At move 11 look at knight to f5 instead of bishop to f5 the bishop is trapped because if it moves black loses the exchange. Then bringing the queen to f3 to try to get an exchange of queens on f3 ie queen takes on f3 then knight on f5 takes queen. The idea is that both blacks bishops and his queen are all set to stage an attack on your king side. So you try to exchange off the pieces to at least disrupt the attack.

First mistake and what lost you an important tempo in the opening is the fact that you first pulled the bischop back to a4 and then after the opponents next move captured the knight anyway. Why not capture right away and get an extra move or keep your bischop on the nice diagonal and develop another piece, your knight maybe?

Although playing Ba4 followed by Bxc6 looks strange, there is some logic behind it: Black often plays f6 in some lines in the exchange variation, so waiting until Black plays Nf6 before exchanging has a purpose.; It's not just a loss of time. But I don't think the "Deleayed Exchange" variation is considered that good for White

I think you gradually drifted into a bad game. I think 7.d4 was better than 7.O-O. And 8.c3 followed by 9.b3 seems very slow. 11.Bg5 looks aggressive, but it leaves you with a Bishop on g5 that needs constant care if you don't want to lose it. And 12.dxe5, inviting his N to e5 just makes things shakier. I think 12.Nbd2 was safer.

15.Nh4 is just a poor move. Your position is uncomfortable now, but 15.Bh4 would have been better, although after 15...Bd5 Black's two bishops rule the board. 18.Qd2? is the kind of move we end up making when all the "good" moves lead to a bad position. It's a blunder, but it's the kind of blunder we're all familiar with from our own games (I know I am). When evey reasonable move leads to a terrible game, we often end up making an unreasonable one.