Series of blunders

Actually I thought

14..Ng4 15.Bxe7 Qxf2+ 16.Kh1 Bb7

0-1

or atleast saving my bishop if opponent found this. Completely missing 15.Bxh7+! despite thinking for 2 minutes

12...Nxd4? was falling for a trap.

In the Slav, I was expecting Black to challenge the center with a c-pawn thrust - usually to defend the d-pawn under pressure of White's c4, but your Nc6 development forfeited that option. On that note, perhaps declining the gambit was a bit better than accepting it, as it commonly makes for a more interesting game. The second move I don't like from Black is 4. ...e6 - you hem the light-squares bishop behind the pawn line, you're supposed to improve its range before developing that pawn, or White will explore this deficiency later on in the game (considering it reaches that far ahead). Since you didn't unleash your LSB, White builds a delicious center, which would be quite tough to challenge.

Not harrassing the bishop as soon as it got to g5 was not ideal also, since you didn't attempt to reduce its range with h6, but the losing move was indeed 14. ...Ng4: removing the knight allowed the Bxh7+ check (the knight was guarding it), your left an unprotected bishop behind just for grabs, your queen was left in the middle of the board also unprotected and you didn't have time to finish development when those complications arose. Maybe 4. ...Bf5 would have avoided most problems, who knows?