[TimM]

Now is the time for 18...d4. We won't get another chance after he goes Bb2.

Just on general principles:

We are giving up a pawn with this move, but the pawn we are giving up was weak and subject to being lost anyway, and the pawn he gets in return on d4 will also be weak.

His bishop, which we were once worried about becoming strong, becomes blunted by his own d4 pawn, and there are no longer any targets for this piece. Our knight and queen gain the nice d5 square. We open both of the files that are rooks are already standing on. In some lines they tie down the white rooks by penetrating to the second rank.

This move ...d4 changes the whole character of the game, from one where we are suffering to defend an isolated pawn endgame, to one where we are taking control of the game with our active pieces.