This is what I call a "lucky win".
Or maybe I shouldn't label it "lucky", but "blundering with a purpose, the purpose being to make him overconfident"? Yeah right.
Yes, I think your post #3 is it. Despite your mistakes, the most important thing in this game (e7+) you saw, and he didn't.
Actually, it looks like black can at least try 20...Be6, connecting his rooks and so preventing immediate mate. In fact I'm not quite sure what happens there, but it's definitely a good try.
As Elubas said, 20...Be6 21. Bxe6+ Kh8 22. exf8=R+ Rxf8 and you're amazingly a piece up, but with a wreck of a position. 23. Rf1 or something might defend... 23...Rd8+ 24. Nd4, and it's hard to judge.
You spotted most of your blunders quite accurately. Just one improvement to suggest: 10. 0-0 then Ng3 or f3 seems safe enough, rather than 10. g3 weakening the kingside dark squares.
FancyKnight wrote:
I wouldn't have resigned.
Youre right, Be6 and it isnt mate cos he cant get a queen cos the second rook on the last line. Good seen
5. Qd2??
In a real fight Queen vs Bishop, maybe the queen will win. In chess, this move tied up your queen to a bishop.