Everybody tells you to play longer chess games to improve - but very few people ever do it. I was that way and went full 180 and here are some my experiences and what I learned.

First, I improved my chess quite a bit and  I get way less frustrated. I enjoy chess more. Recently I even played OTB Rapid tournament (30 min per player) and I am going soon to play my first classical.

About playing blitz now

At this point I don't even really want to play blitz, anything <5 min especially. Now it may look arrogant, but it is really (at least for me) 95% just mindless intuition piece moving hoping to get that dopamine rush from the win. You improve very little and slowly - blitz has its place - but don't expect big returns improvement wise quickly or like ever.

How to find longer games - ChessDojo

There are random people playing longer games, both on chess com or on lichess, I don't play other platforms. That is enough for the start. If you want to get tad more serious, try joining clubs (same platforms) or try ChessDojo. Just google it, they have their own training program and free to join chess open classical tournament where everyone can join, pretty much anytime. I filled simple form and I get paired with someone close to my rating - so far I have played 6 classical games. Opponents there are a bit more serious, so there's extra challenge. Another thing is schedule - even if you don't feel like playing - there's that one ChessDojo game weekly that keeps you active - it helps.

What time controls

For casual play 15+ minutes is minimum I would recommend. I feel 10 is way too short. Optimal, depending on level, but I found that I like 30 min and 45 min for amateur levels. ChessDojo minimum allowed time control is 45 min + 30 sec increment. These are time controls that you have plenty of time, but the game is not way too long. Some of these are technically rapid, but for all purposes for amateurs, its like classic. Now if you are really serious, 90min is what you play and that is what ChessDojo recommends for higher rating players (1600+). I had for example trouble concentrating playing real 90 min game - getting impatient and playing quickly.

Main advantage - seriousness

Since you dedicate yourself to such a long game, you tend to prepare a bit. You know your opponent, or at least what to play. You check that opponent plays certain opening, check out the lines, look at plans improve and see how to play against it. Then, in game, you practice to apply what you learned, time management and pressure, especially if result matters to you. Now for next game, you prepare something else, so the next one and soon after you learned to play 6 different lines. Longer games cement these lines better in your mind and they actually stay there. I can't remember like none of my blitz games.

Analysis

Then, of course you should analyze your game, and since you had time during game, you really know at what parts you struggled to find a move. And if you analyze those positions, you won't forget it for long time. The more serious analysis is, the more you improve. Its kind of tempting to just turn on engine and be done with it in 2 minutes. That is one of the reasons I started my Youtube channel, by sharing my games I force myself to analyze them better, I connect more to chess world. And I was always interested in instructive aspects of the game - so if I can help someone play chess better - I am happy to do it.

What I learned so far

Game 1 - ChessDojo Rd.3. How to play English game (I joined tournament from Rd3)

- Learned way more about English opening, from prep and this game. Later I had really good game vs 1900+ FIDE rate player in my OTB tournament, that I just lost in the endgame due to time trouble.

Game 2 - ChessDojo Rd. 4 - King's Indian Defense - plans, mistakes and activity

KID defense is really hard for players <2000, pieces are locked behind pawns, lots of hidden movement and obscure errors - but the play can develop to most various lines. I learned new line here, lots about plans and where to play analyzing this game.

Game 3 - ChessDojo Rd. 5 - fighting with Kan Sicilian

This one was tough, thought me to appreciate hedgehog structure and understand d6 better. Also the important of time management and pressuring opponent that way.

Game 4 - ChessDojo October Rd1 - Kings Indian Defense - Tactics and blunders

This one reapplied my knowledge - as I play KID game already. Now I played another line, but similar theme was there. I understand now for example how to tackle black's typical f5 push much better.

Game 5 - ChessDojo October Rd2 - Closed g3 Sicilian - Time management and preparation

This one practically thought me the important of prep and time management. I had first 11 moves in my prep and I stayed very comfortable. In the end, I practically won the game on time management alone, as the opponent blundered.

Conclusion

Each of these "classic" games is unique experience and story in itself that stays with me. They were all fun and interesting, even when I was tired to play, and overall they improved my play visibly. I recorded 15-20 minutes for each of these games, if you are interested for details for some of them the link is here. This is the link from my first ChessDojo game and in the playlist you can find others as well.

Good luck playing and enjoy your chess!