4...f5? I don't understand this move. You weaken your kingside critically just to get a brief central attack. ...g5 would have been an improvement.
5...Bd6?! Inhibiting your own development is mistaken when playing against a gambit! Since you end up giving back the pawn anyway, ...Be7 would have set up a nice defensive formation. If you wanted to hold on to the pawn, 5...fxe4 6. dxe4 g5 would be something to consider here.
10...Bg4? This bishop is just a swinging bishop since White can immediately break the pin without weakening the king. ...Qe7 is preferable, when ...Be6 removes White's dangerous c4-bishop.
12...Nxg4?? The final error. Black gives up a piece for no compensation and now the win is not very hard for White.
Objectively, White shouldn't have enough compensation for the gambitted pawn, but Black's position is quite dangerous to play and one slip can cost Black the game. If you don't like to take the risk of accepting the pawn, you can decline the gambit with 2...Bc5, become the gambiteer with 2...d5!?, or use GM Tony Miles's [unusual] idea of 2...Nc6 3. Nf3 f5!?
here a game that i lost in it :) (i played the black)
please check it and give comments about my gameplay - for my improvement (i want to learn from my mistakes in it)
thank u!