I played this game against a good friend of mine recently. I know I blundered an advantage in the middlegame, but I can't figure out where exactly I went wrong. I have some idea, but any input would be greatly appreciated, especially in regards to my 29th move. Thanks!

"11. f4 I'd like to be able to open up the center, then (depending on where black's king goes), launch an attack."

Sounds like a reasonable plan, why not follow up with this on move 12 and play 12. e5.

The f6 knight doesn't have a good spot to go. For example, 12. ... exd5 13. fxd5 Ng8 it's quite unpleasant to put the knight back there. But 13. ... Ng4 gives white two ways to win, 14. e6 Bxe6 and now either:

15. Nxe6 Qxd1 16. Nxc7+ Kd8 17. Nxd1 wins a piece or

15. Rxe6 fxe6 16. Qxg4. Looks like a potentially deadly attacking position.

Looks like all this same stuff is still available after 12. Bxc4 bxc4 13. e5

23. Re4 and Rg4 was a good idea. But maybe you can involve the other rook immediately with 23. Rd1 followed by 24. Rd7.

As in all  game at our level, there are several inaccuracies, but it was still a live game until 29 Kh1? Then the back rank weakness was fatal. Here are a couple of other observations.

10. Re1 was a clear mistake and fell into an ancient trap. After 10...Nxd4 11.Qxd4, c5! the B on b3 is trapped

22.Qe4 was a strong move and preserved a nice edge for White

27.Qg3 was not best. 27. a3 was better, I think. By the same reasoning, Black should have played 27...Rxa2

A hard fought game, and nothing to be ashamed of at all. I hope it will increase your back rank conciousness