Wow, this is eerily similar to how I was. When I was like 1300-1600 USCF, I would always seem to know more than my opponents did. Yup, they would play Bg5 and then want to take on f6 because it was fun or something. Yet I would still lose to them half the time! Well, you gave yourself a good self-diagnosis: find the simple tactics. Because if you don't, you will erase many moves of hard work and smart decisions. It's as simple as that. Work on tactics. I used to dislike tactics and only like positional play. But one day I decided to get really passionate about tactics, take pride in every fork, pin, or sacrifice that I found. Once you get enough patterns, you won't miss simple stuff. And you'll often be able to win much more quickly and easily. You have a lot of potential, because you're doing well at the complex things in chess; now you just have to work on the simpler things :)
Regarding the ending at around move 29 I'll say a couple of things. First, you don't need to rush with the b pawn. You have all the time in the world to get your king in the center, gain space with your pawns, at no cost. That will only increase your chances of winning. Still, I guess here pushing the b pawn was ok, since on b2 you can always protect it, but when your opponent is helpless, it often doesn't hurt to improve your position before trying to do anything -- why not?
38...Rc1 keeps the rook in a good position, tying down the bishop and knight. 43...g5 is not the best because you are trading off pawns. The less pawns on the board the less targets available, which is not what the stronger side (you) wants. A plan like 43...e5, moving your king up to d5 maybe, pushing ...e4, stuff like that will really increase the pressure. You might even start attacking white's kingside with ...Rc3-f3, because why not grab more pawns? The more advantages you get, the harder it is for the opponent to try to swindle you, so it's always smart to get as many as you can.
Once you traded off too many pawns, you reduced your options. On move 55, you still have the b pawn, but that's it. All white has to do is stop the b pawn. If you had won some of white's kingside pawns instead, or tried to get a passed e pawn, white would have been overstretched trying to defend things from all over the board. I still think tactics are the most important thing by far, as your opponent simply does not deserve to win just because he found a fork or two, meanwhile he otherwise got outplayed :) Still, if you simply let your opponent do this, they will take advantage of you.
I just played a live 30 minute game that seems to follow a pattern with me - I demonstrate stronger chess ability than a given opponent throughout the game...and then make a series of horrendous errors that omit anything and everything I did right. Take a look -
So there you go - there's not a lot of mystery as to why I lost this game - I made a bunch of really stupid moves in a won position......what do I do about this? I am getting killed by the most braindead of counterattacks while making right decisions that are much more complex......it's painful....so help Alison with her terrible moves.