Help me, I lost this ending.

as @l8pawn said, you traded your way into a strategically lost endgame. The reason for this (in general) is because white has a 4 on 3 king side majority so he will be able to create a passed pawn on the kingside. On the other hand, black has a 4 on 3 majority on the queenside but due to the double pawns, black will be unable to create a passed pawn (at the very least it will be very difficult).

Edit: while the ending may not be completely lost, it is very difficult to hold in practice

@bugcat2 The thing is: with after 4. Bxc6, black can play Bd6 to defend the e-pawn in 1 move (the e-pawn can't be captured immediately with 5. Nxe5?, but after 5. O-O, white threatens to capture the e pawn.

With 6. Bxc6, the Bd6 move will be awkward because it will require a second tempo to move (as it was already moved to e7)...

So white's loss of tempo matches with black's loss of tempo.

Thanks for the response but I don't really understand, @SavageAntarctican .

If 4. bxc6 bd6 White just retreats the bishop to A4 or D5 and is up a piece?

@bugcat2 Let me show you the difference:

I see now, thanks c:

This thread always reminds me of Joni Mitchell.

I have a game which suits this thread just right :(