What could I have done differently?

You missed simple tactices in at least two occasions. So my advise would be to practice more tactics.
If you have already studied a book that explains the tactical motivs in chess then just do more puzzles.
If you didn't read such book, then you really need to get one!

11...Bb7 is passive: more active 11...Qa5 driving Bg5 back.
13...Nxe7 is wrong as it loses the pawn and yields the d-file. After 13...Qxe7 you can hold.

I would use the stockfish post-game analysis feature and study the game move-by-move... try replaying various continuations from position... If you get into the habit of studying your losses and learning from those studies you won't need to search for answers on the lichess forum. BYE-the-WAY: Your opponent Nai-Harn is a player you can learn from... Study his games and when you are ready challenge for rematch. GL.

13. Qxe7 is what I would play. Knight pretty hot there.

Also missed by both:
If white plays 19.c3, you lose your bishop.

Besides, rook + bishop vs queen is not exactly losing man. Play on!

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