There are a lot of complex answers to this, but I am going to try for a simple answer based on the efficiency priorities from the different stages of the game.
So, when you are playing in the Opening, the priorities are
approach unclaimed corners
approach an opponent’s corner or enclose your own corner
create or prevent large extensions
create or prevent small extensions
once all the corner/side territory is claimed - make vertical moves into the middle
However, the transition from Opening to Early Midgame is not always smooth or easy to see, and a lot of players start taking advantage of Midgame efficiency priorities before the Opening is completely over.
In Early Midgame, one of the simple priorities are to push into the middle from both sides of a large extension to expand your side potential to create a big moyo - and conversely, to make moyo-denying moves (shoulder hit, big jumps, etc) to prevent your opponent from making a moyo.
The way I think of it is
in the Opening one is focusing on 1 dimension (length) - “how much of this edge can I grab so I can make a large extension”
in the Midgame, one needs to start thinking in 2 dimensions (length x width = area) - “how can I push inward from both sides of my large extension to make a moyo”
Now, of course it’s not always that simple. With higher level players, the first 50-100 moves might be a life-or-death fight for one corner that slowly spreads across the board before they even “finish” the Opening. Other players will make moves that result in their opponent making weak groups that need to run to reinforcements, and then they use that direction of play to create large extensions or expand moyos, etc. So yes, there is a lot more to the story than just following the efficiency priorities, but I feel it’s a good tool to use to put those other strategies in context.
This article I wrote might be re-hashing stuff you already know, but hopefully there will be some useful nuggets of info in there:
I had to look at your stats, you’ve played almost 7000 games (!) but only 276 games on 19x19. So I repeat the obvious advice: play more games on 19x19, review of games with AI or with a stronger player, watch professional games.
Also, I’ve had a quick look at a game or two, it seems that you need to work on making better shapes.
Judgeing from this game Freundschaftsspiel, compared to your stronger opponent you seem to be doing well enough in global play, but when the fight starts involving cutting points and liberties (as would happen when the board fills up), there are multiple instances where you suddenly collapse for no clear reason, as if you are confused about what is going on.
Like others have said, I suppose you need more playing experience on 19x19 and try to follow the decision process that @Animiral posted.
This suggests that maybe you are not ready yet for 19x19 and need to improve your fighting skills on 13x13 and 9x9.
Maybe ask if a somewhat stronger player wants to play a teaching game with you.
You know already what your areas for development are. Your opponent can focus in on that.
Have fun playing go and don’t take it too serious. It is only a game.
Yes, I played mostly 9 x 9, then 13 x13 and started with 19 x 19 at Feb 2021 ( when I finished G(U!)O Juans Basic Step by Step course) . So yes, my experience at 19 x 19 (compared to the the smaller board sizes) is small. I am trying to play 1 or 2 games a day , most often sth like 10m + 3x30 byomi.
I just began the topic because it is a little strange for me, that my opening (the often said most difficult phase of the game) seems quite ok while I then suddenly break down. An effect that is much more feelable than on 13x13 where I keep my level. However I ll play more, do my exercies, stay patienzt , ommmm
( I am a non-golden site payer and have an good graphic card , coming from chess -> so I am used to analyse quite a lot with AI)
i dont improve on 9x9 and 13x13 much more and it got a little “boring”, that s why I started 19x19 (the REAL game ) this year. I am much too old to be too serious but since I played chess quite seriously it seems to be my nature not to be able to just play and accept when I see how far away I am… (10 stones exactly )
I’ve not looked at you games but what helped me get over a hump was really thinking about strong and weak groups. Which of my groups are strong or weak, which of my opponents are? How can I strengthen my weak group so that I also weaken an opponent group? Are my stones connected? Are they still connected as things change? Does it matter if they get cut?
Reading is probably actually the answer but I’m too lazy to do that so I try and think about these questions instead.
And thanx, the move-finding-algo seems to be comparable to chess, but the expected results are sometimes hard to calc especially when low on time. Of course there is also the lack of experience, but no 1 - 3 sound familiar and when I have time I think that way… my reading is (I am OOOLD) not the best anymore, also I guess in faster games it s more about shapes, experience, …
I’m not very good at chess but my opinion is in Go we rely more on shape intuition than chess. Since you are a strong chess player I assume you have experience calculating deeply. Hence here are my advice.
Timed puzzle. Work on puzzles that are a little easier than what you normally do and don’t spend more than 30s on each. Try to learn the shape instead of brute-force.
Focus more on tesuji rather than life and death. Tesuji applies to a variety common midgame situations, such as cut, connect, sabaki, invade, while life and death is just life and death.
If you have chance to consult a strong player about a game of yours, ask them about moves where they think A is the only move without even thinking but you played B.
Just my suggestion based on the info I can see from the post. May or may not work.
Btw I think opening is the easiest part of the game, as opposed to midgame and endgame.
Edit:
Whether opening is the most difficult part of the game depends on how we define difficulty. It’s extremely hard to rigorously prove a move is optimal or not in the opening, because to do so we have to go over an astronomical number of variations. To prove an endgame sequence is optimal is easier, at least for some positions. So in that sense opening is difficult indeed. What I mean when I say opening is the easiest is, in opening, you won’t lose much if you do not play the best while in mid game we humans constantly make 20 points mistakes.
You could try watching videos by strong players who explain what they are thinking about whilst playing a game. Then you get to see application of the principles people can tell you about in theory actually put into practice in real game situations where your opponent does annoying things real opponents do. Here’s one of mine with some midgame fighting. Safe fighting: Fox 3d Game 4 - YouTube
I’d also recommend BadukDoctor’s comment whilst play series as he makes a side board to show variations, so that particularly helps weaker players. (And just to say I don’t recommend dwyrin before others chime in ). Ryan Li and Haylee good too.
Thank you! I am actually doing the things you suggested , maybe I am a little too impatient.
@chess and shapes …
yes and no I have played on tournament level (around 2150 - 2200) for almost 30 years. As the experience and knowledge gets bigger you tend to calculate less and less and rely more on more on “shapes” although they surely differ in nature from go. So when I read people “read” 20 - 50" moves in go I am astonished. Currently this is far beyond what am “seeing” … a chess GM once answered the question “How many moves do you see ahead” … “One, sometimes two… but most often they are the right ones”
@yt BadukDoctor, Ryan Li and dwyrin ( ) 100% agree hehe
There are quite tons of useful information on inet, compared to the “old days” its maybe almost a little too much. I also have abos for Redmond, AGA, YeonWoo, Yoonyoung Kim, Yoon`s Baduk Cafe, …
I think reading deeply is easier in go than chess, see:
Also the idea of a strong player not needing to read so far, because they have a highly trained intuition and experience that makes them more likely to read the good moves applies in Go. A strong player can read deeper than a weaker if they need to, but if you have a pro playing on 1s instinct so barely reading at all, and a 10k being allowed to read 20 moves deep the pro will still destroy the 10k. Bots are similar, their instinct is superhumanly strong, but there are some positions (e.g. complicated capturing races or ladders) where instinct isn’t enough and you do need to read, and these are the kind of positions where a human with lots of time can play better than a bot with little time/resources. They are getting rarer and rarer though as the bots improve.
). Ryan Li and Haylee good too.
) 100% agree 
hehe