3 e4 - I won't say it's bad, but you are creating a weakness on d4.
4...d6 - He really didn't have any other good move to protect the pawn. Qe7 would also have blocked the dark-squared bishop for him, and Bd6 would have blocked in his light-squared bishop pretty badly.
6... h6 - I'm guessing he's trying to keep your knight or bishop off of g5. I don't think it's really the best move, because what would you do once you got there?
9... Kh7 - I have no idea either.
11...Ng8 - A bad move. I guess he wants to move his bishop out that diagonal really really badly. But he could have at least moved it to d7 or something instead of the back row.
15 dxc6 - I don't know if I would have done that. It lets him develop his knight for free, and to a spot where he can later put it on that really nice d4 square. Luckily for you, this guy doesn't seem all that interested in development.
19 ...Na6 - Wow, I haven't seen knights that bad in a while. You've got a pretty significant positional advantage at this point.
25 d8=Q - Yeah, this pretty much throws away your advantage. It's more or less even after this.
42 Kg2 - No, it's not dumb to get your king involved. You probably should have done so sooner. But I would have put it on f2 instead. It gets your king closer to his dangerous pawn, and keeps his king from getting any closer.
45 Kg3 - Should have played Kf1, getting you closer to his dangerous pawn and preventing him from checking you again with his knight.
46 Kg2 - Yeah, by now it's too late to stop him from queening, so you needed to make sure you could get your own queen.
48 ...Qd8 - He could have just played Qg1 checkmate.
This is a game where I felt I was slowly building positional advantage against my opponent, and I feel like I missed a move or tactic along the way that could've cashed in my advantage. In the end I lost, though I THINK I should've won, was this indeed a game I should've/could've won?
More than anything, if yall could just look through the game and read the notes along the way, then tell me if I'm in the right headspace. I am still a beginner so I want to make sure that I analyzed my game correctly and didn't miss anything.
And if you notice something that I didn't please mention that as well, thanks guys :)
P.S. Happy 4th of July fellow Americans :D