Game anaylsis.. mistakes, miscalculations?

This is a game I had about 5 minutes ago. I'm not really sure what exactly I did wrong. much help and analysis appreciated

19.Bxe5? (19.Rxe4 and black cant get his knight to f3, also protects your bishop on c4. after this move your queen to a nice square and double rooks, you should be fine.) 

Look at the position after 4 moves...black couldnt have played more passively.  5.  d4 is correct here, not d3.  The threat is pxp, pxp, qxq, black retakes, nxp -- winning a clean pawn.  Black must therefore give up his central pawn, centralizing your knight in the process.  

Grandmasters sometimes will violate rules by doing things like your 5. d3 8.d4, but the rest of us should avoid moves like that like the plague.  Normally, its just a waste of a tempo.  

Look at the position after move 11...black is at least equal.  

Anyway, there are a lot of tactical things later in the game, but what I am pointing out is I feel something more concrete that can be learned.  If black wants to play passively and hand you the center and more active pieces, you take advantage.  

Best in your further development! 

18. d5?

As a result of pushing pawns in front of your king you have weaknesses there that can be attacked. With the queen alone, the attack won't go through. 18. d5 allows black to quickly bring a knight to help the attack because you give up control of the e5 square. Better here is probably 18. Bf1 Qf5 19. Bg2

19. Rxe4 looks like it loses pretty much instantly to ng4. 

Its looks like 18.d5 was your downfall. Before that the knights were kept out of c5 and e5, and you had good square control because of your centralized bishops. It would have taken a lot longer to maneuver those knights over to the kingside. It probably wasn't a loosing mistake but I think after that you just lost the initiative and your game got a lot harder than it needed to be.