Why was this position losing for me?

On this move, I thought I had a decent advantage. I had planned on attacking his king. My pawns were pushed forward, yet the position shows that my opponent has an advantage. How come?

Take a look in the analysis
Then you might see what’s wrong

Your king was lonely.

The B file is semi-open with a rook one move away, and your king must have been feeling very drafty with all those open doors. Your opponent didn't capitalize on that.

King safety is the #1 criterion when evaluating a position. The white king is unsafe while the black king has castled to safety.

Yes, your pawns are advanced but you are basically playing without your A1-Rook and C3-Knight. How are you going to attack black's position? Usually with pawn-advances like f5 or/and h5. f5 just loses a pawn to exf, gxf and then either Bxf5 or Nxf5.

So that leaves only h5. Black wont take, so you will have to take on g6. h6 is silly/bad, because then you got nothing as f5 is still blocked and g5 is even worse.

So now you take on g6 and black takes back, let's say with the h-pawn. Now imagine you can just put your queen on the h-file. Black needs to play only Re8 and your threats are all for naught.

Your attack may appear strong at first glance, but that's as far as it goes. And here I haven't mentioned the counterplay black has during the maneuvers you need to prepare any kind of an attack.

Overstretching. Negative space "advantage".

You must be able to control the space!