nagging suspicion 😔

So my SO and I have been dating for 9 months now and I've previously caught him romantically texting this other woman and he said that he was only texting her because he thought we were not that serious yet but promised he had ended things with her. In my gut I had a feeling they were still talking but he hid everything well and I had no evidence. I went through his phone again and found this convo he had with a guy friend of his. What do I do?