Need deeper explaination for this game (~1300 elo)

This match would've gotten me to 1300 elo, but I lost it. I'm learning how to calculate better and I did my very best, but something didn't go right. The reason why I am creating this thread is that after the analysis I don't understand some of the missed concepts and I would appreciate if someone else explained what went wrong there too, as well as how can I avoid making these mistakes in the future.

I added the game and comments that explain my processing during the game. Thanks for any feedback in advance.

You annotated a lot (which is great!), so I'll just comment a few moments.

9...c4? This move is a positional mistake for the opponent. Your comment mentioned strengthening the pawn chain. Did you mean c4 and d5 together? This is not stronger. Pawns are best next to each other with the ability for either pawn to advance one square and be defended by its neighboring pawn. The ...c4 move is a mistake here because it gives up the tension in the center and once pushed, then black never has the option of ...cxd4 and white maintains better central control.

18. Rac1? You were on the right path, but Rfc1 was best because it has the benefits of your move, but a2 remains defended.

21. Nf4! was strong because it attacks d3 and hopes to pressure the outposted bishop. The engine continuation also avoids losing material from the c2 knight fork. The line goes 21. Nf4! Nc2 22. Rxe8 Rxe8 23. Rd1 (again, chosen to pressure the d3 bishop).

24. Rd1 would pressure the d3 bishop and Rc1 would be a fine alternative too. Playing Rb1 (as in the game) looks unecessary because it walks into the bishop discovered attacks after something like an immediate ...Nd4.

I hope that helps

Thanks for your feedback.

Is there any way I could avoid making these calculation mistakes? I've learnt that I should focus on making attacking moves and trying to go into the enemy's territory if possible, and I knew that the opponent should do the same so I have to predict these moves. Honestly, from just that tip alone I felt more confident at calculating positions, but I may have forgotten about how to defend in a losing position. Maybe you know someone who made a great video about improving calculation in chess?

Here's another match that I've played today. COMPLETELY fumbled the midgame here. Same question as before, I'd really appreciate if someone explained to me missed concepts along with how to avoid making these mistakes. I might dedicate this whole thread for my lost matches.

Thanks in advance.

I don’t have time to go into the game in depth but The reason why 26…Kh8 was better is that after 26…Kg8 you can play 27.Bxd4 Qxd4 28.Re7 which wins. 

But in the similar position with his king on h8 he can check on d1 with his queen, then after Kg2 (your only move as Kh2 Qd6+ hangs your rook) he can check again on d5 followed by bringing the Q back to g8 (that square now being vacant) which holds the position together for him - just 

Actually having checked my previous comment with the comp to make sure it was correct it points out he can also just play …Rg8 as well - but either way …Kh8 is to be able to defend the g7 square more easily.

Calculation is a skill and something which you gain through pattern recognition (solving puzzles and playing games) and through conscious calculation practice (especially endgames or endgame studies).

Feel free to check out my YouTube channel for any chess topic. As for calculation, I have at least two videos you may be interested in. The first one is the "secret" to thinking 10+ moves ahead and the second one is my recent video on puzzle rush, so puzzles like in the video may help your calculation practice: