Hello,
There was nothing to worry about with 3e3, it looks more like lack of opening knowledge than anything else. 5...Nc6 was first slight mistake maybe, because want to have pawn break c5 later. So 5..Bd6 , followed by castling looks better. Also, when finished development, chould think about exchanging bad bishop, which your opponent has helpfully made available. 7...Ne4 is helping the opponent exchange badly placed Nc3, 7.0.0 looks ok. 14 ...Bb4 and getting your peice trapped was horrible. Again just castling would of solved immediate tactical problems, you are still worse because of bad pawn structure.
Do you really mean 18axb4 was a mistake by white? He gets 2R + B v Q. Afraid think was dreaming if thought was ever winning game.
On the plus side, don't think you are too far behind opponent. Just basic development, not leaving king floating about in centre for no reason, and keeping option of pawn breaks would have given this opponent something to think about.
I played this game when I was solid in the 1600 range and wanted to see if I had what it takes to be a 1700. I didn't, and I would like to know what exactly was lacking in my game, because I can't help feeling there was a victory somewhere in here.