boyfriend's stupid ex

hi I need advice, well basically my boyfriend's ex messaged me because she reckons he cheated on me with her when we first got together.

anyways when I asked him about it he denied it and says it happened when we first split up and that she's trying to split us up again.. we have a son together and he has a son with her but she now has a boyfriend and a baby with him, so don't understand why she's trying to do so... one thing comes to mind, she's a jealous phyco

but since then it's caused problems in our relationship.... like I trust him etc but I can't stop thinking about it and it causes arguments, has anyone please got any advice on how to put it at the back of my mind and forget about it? I love him so much and don't want his ex getting what she wants by us splitting up? when we drink it causes arguments then because i end up bringing it up and before it happened we never argued when we drank etc, would really appreciate and help and advice, Tia!