Black a-rook is coming to the c-file soon, probably even next move depending on what we play, so our decision should take some prophylaxis into account concerning the consequences of pushing the c-pawn.
Just found out about the game. I hope nobody minds having another player.
I recommend 15. e4
e4 will eventually become weak after after Rac8 and c4 because e4 lose will lose the d3 pawn as a defender. After black plays b4 our knight on c3 will not be able to defend e4 either.
My idea is that we can still play e4 now and meet Rac8 with Re1. Then after he plays c4 we can play dxc4 (or even d4) and we still have our pawn on e4 sufficiently defended. If you don't like the idea of moving our rook again we can play b3 after Rac8 instead of Re1
For me the important thing is that we find some way to advance in the center. Black seems to have an advantage on the queenside already so we need to do something active to build counter chances
