Some insight on my game!

Here is the game I played against SF level 6 on lichess. The first 9 moves are from opening explorer. From that point onwards, I find myself losing initiative almost immediately to never regain it back again. Forget winning, I tried hard to defend but eventually capatuilated in the end. I know computer analysis is available, but I need human feedback (preferbly from over 2k rated player) on how would I have dominated the game. Thank You.

I don't understand 15. f3 at all!

If anything it seems like the pawn would be weak/overextended on e4!

8 Nxd5 just help its center control. Although I understand that the threat of Nxf4 might make you think that you are saving a tempo.
9 Bd3 would be more active. Gives black the choice of Bg4, pinning, or Bb4+, forcing the retreat.
12. a3 is too slow. Thinking about a minority attack, Rb1 would be more useful.
14. h3 is a too common mistake. It does provide luft, but gives away more of the initiative. Better is b4, forcing some weakness.
15. f3 weakens e3. If you could push e4, then this would be a good move. If Be4 was a threat, it's better to play f3 after the move to gain two tempos.
17. Bd3 leads to the ruination of your pawn structure. Bxd6(Be5) was necessary. Your e3 and dark squares are weak, but black has weaknesses on the queen side too.
20. f5 is weaker than Qd2. Nc5 trying to force a weakness or a defensive piece is possible.
[Stopped looking.]
This is just a quick review, so I may have missed simple tactics. Although I hope that other players comment with their clearer opinions. feel free to question any of mine.

I did not analyze deeply, but I agree with the friend above.

-Nxd4 only helps black. It would consider playing Bg3, keeping pieces and threatening e4 in the next, followed perhaps by Nxc3, bxc3 and White has a strong center (similar to Grunfeld Defense, but more blends with Black having played g6, instead of e6), plus you to have helped Black liberate the bishop of c8 that it would take a long time to develop.

-a3 is unnecessary. If you were to put your tower in e1 could prevent a possible attack of the bishop, however this never happened.

-f3 was precipitated

-Allowing Bxf4 was the beginning of his defeat.

You can't post in the forums yet. Play some games!