Need some advice

So my boyfriend of 4 years admitted to hiding and deleting messages from women from me... he says he deletes them to avoid conflicts like this one.

I was laying in bed and heard his Apple Watch vibrate so I look and discover messages from a woman from work. There was a selfie of them two at a company party, messages about being “naughty” from Christmas and messages where he was comforting her when she was “having a bad day at work” so I went through his phone and the messages weren’t there......

I asked him about it and he denies anything is going on. Actually tells me this woman is a lesbian and the naughty jokes were about customers.

I’m so conflicted. He’s cheated on me before so I kinda get the hint. But I have no solid proof.

It sucks ending such a great relationship off assumptions but I need some opinions.