Not sure if 31 f4 is a mistake, but I think it would have been easier to first take a move to do something about the safety of the queen side pawns. Maybe 31 Rd3. 33 b4 looks bad to me. Instead of 34 Rc1, I would have played 34 Kf6. Instead of 37 Rc6+, I would go with 37 Rd1, so that the king could continue against the f7 pawn. I think White is doomed after 40 Rxb6. Maybe play 40 Rc4.
Need help to become better

42.Kb5 is not a mistake since the position is lost anyway after 42.Kb4 Kb7 43.Kb5 f5!
Absolutely, just trying to get across that fact that not understanding why you're not improving while playing nothing but fast time controls isnt working.

In addition to the excellent annotation by DierdreSkye, it's clear you need to study endgame technique, some of which I'll address first, as well as needing a middlegame plan, which I'll address at the end.
You basically lost your last game because of poor endgame technique.
It is easiest for a Rook to attack Pawns from behind them, yet you made no effort to prevent your opponent's Rook for doing it, playing 32 Kf5, and made no attempt to do it with your Rook.
Once you were behind a Pawn, you allowed your opponent to create and Outside Passed Pawn (40...cxb6) that basically sealed your fate. You also don;t seem to understand the power of having The Opposition. Get a good endgame book, like Silman's Complete Endgame Course, from Beginner to Master and STUDY, don't just read it. Learn it well enough to teach others.
As far as the middlegame goes, you don't seem to analyze the position and put together a plan. I recommend the following: when I coached a very successful, multi-year county champion and state scholastic team trophy winning high school team, I wish I had available Fred Wilson's very excellent book, Simple Attacking Plans where the 4 principles are demonstrated by 36 annotated games:
I have come to believe there are only four essential, even primitive, concepts which you must learn and understand in order to play successful, attacking chess - Fred Wilson
- In the opening, whenever justified, relentlessly attack the weak squares f7 or f2.
- Most successful kingside attacks are directed against the squares h7 or h2, and they are often preceded by eliminating or driving off its defender. Corollary: most successful attacks require a long queen move.
- If your opponent’s king is trapped in the center, make every reasonable effort to open and dominate the e-file, and sometimes the d-file also.
- If possible, point all your pieces at your opponent’s king.
I also recommend: Martin Weteschnik, Chess Tactics from Scratch, not only examples, but principles behind how to create pins, discovered attacks, etc. Plenty of diagrams so you can follow in book alone.
If I was still coaching good high school players, those three books would be required reading.

Your first mistake was using those cartoon looking chess pieces that were drawn by a three year old, lol.
… If I was still coaching good high school players, those three books would be required reading.
Silman's Complete Endgame Course
http://theweekinchess.com/john-watson-reviews/theres-an-end-to-it-all
https://web.archive.org/web/20140708103149/http://www.chesscafe.com/text/review594.pdf
https://www.silmanjamespress.com/shop/chess/silmans-complete-endgame-course/
Simple Attacking Plans by Fred Wilson (2012)
https://web.archive.org/web/20140708090402/http://www.chesscafe.com/text/review874.pdf
http://dev.jeremysilman.com/shop/pc/Simple-Attacking-Plans-77p3731.htm
Chess Tactics from Scratch by Martin Weteschnik
https://web.archive.org/web/20140708091717/http://www.chesscafe.com/text/review851.pdf
https://www.qualitychess.co.uk/ebooks/Chess-Tactics-Scratch-excerpt.pdf
Here are some reading possibilities that I often mention:
Winning Chess by Irving Chernev and Fred Reinfeld (1948)
https://web.archive.org/web/20140708093415/http://www.chesscafe.com/text/review919.pdf
Back to Basics: Tactics by Dan Heisman (2007)
https://web.archive.org/web/20140708233537/http://www.chesscafe.com/text/review585.pdf
https://www.chess.com/article/view/book-review-back-to-basics-tactics



I checked up to first 20 moves only. The major strategic mistakes you did were 13. Bd2?? 14.Rd1??, 15.Nd5??( why u do these moves while ur kt on c3 is already protected. Reasons? No idea to make strategic plan?)
After these three bad moves, you lose all your initiative advantage of white and even reached to inferior endgame with double pawn.
My suggestion. 13. f4!!! Open rook file.
So I'm stuck at 1100 - 1200, and I want to get better. Can someone tell me what I can improve on and what I did wrong in my last game?
Help is appreciated. Thanks in advance!