Well my point is @LukaCro you keep putting it as lack of opening prep. Which it isn't. You're just studying chess. And thinking it is "opening prep" might point you in incorrect directions. If you haven't noticed there already is several missed directions. A lot of people assume you are memorizing moves. And contrariwise you think people think YOU'RE memorizing moves. Both sides are pointed this way by default because of what the term opening prep does to people. If you were looking at what some of us are saying, we are saying "Openings" don't matter up to SM or FM+. The reason that is stated is because you don't need the ECO openings to play good chess. And this is true. You can apply principled chess all the time without ECO type openings. AND if you do it correctly a lot of the times you can come up with ECO moves just following the principles. And it might surprise you how accurate this thought process really is.
By serious analysis I mean taking your games or master games. Putting them to a group of people of ranging strength, hopefully someone way higher than you is in your group somewhere. Spending on average 4 hours on a single game and combing through the actual ideas. I know you didn't on the last game because you focused on the move you didn't play rather than the reason you felt your position was worse, which was you had a backward pawn that you couldn't figure out how to eliminate. And which by the way, even after your move "a5" you still had plenty of chance to cure said backward pawn. No mention of these ideas. Just that you missed one move and blame opening prep. Slav and semi slav positions have MANY opportunities to play c5 to get rid of that weakness.. Your game was no different. Analysis of your games is "MORE IMPORTANT" than analysis of master games. Personally I need this type of idea, but I can't find anyone 2300+ that wants to do analysis group or private unless I pay them 60 an hour. Sadly I even tried getting 2000-2200 players interested and they tend to not want to do it either. I do have A and B players that will sit with me, but I think it benefits them more than it benefits me. I enjoy it, but I do wish I could find more higher.
I think @PixelatedParcel has a good thing going. You could try joining his discord. When I was there it was pretty hopping. Good chance I will sit down with someone for serious if they asked me. Only problem with me is I am sometimes crude and rude, so you have to deal with my craziness if you want me to sit down with you. :-) Our openings might conflict a little though. I notice you take on c4 a lot earlier than I do. I will wait for offers to do botvinnik lines. When I do Meran lines, I am weird and play a6 instead of Nbd7. I am also trying to figure out a new white idea. Took up a chessable video course by IM ChessExplained called "Keep it simple: 1. d4" I am absolutely loving it. The strategic lessons alone are worth the price I paid for it. I have a discord server I am trying to build too, but it's slow going. Would love to have it lively. Also I hear there are others as well. I would probably suggest seeing if any fit you.
