Your opponent played well! Some low rater comments:

14 f5?, sacing a pawn for no gain and bringing his light-squared B into play, was obviously inaccurate. The same applies to the next move (dxc5?, creating an excellent square for Black's Q). Perhaps it comes from pushing too hard/too early, which is something I am keep doing.

Perhaps the patient 14 Kb1 was the go, noting that a draw against such a high rater would have been a great result.