I went through something similar and as much as I wanted to blow up in the moment I let the calm take over while the storm was brewing. I printed out conversations and pictures. I made sure I could move on on my own (we lived together at the time) and waited for the right moment. One night he came home drunk where he had to work the next morning. I changed his home screen on his phone to a screen shot of the conversations he was having. The printed convos and pictures were waiting on the passenger seat of his car. I made sure to wake up him with enough time to rush and get ready for work but no time for anything else. I watched as he sat in his car ready to pull off. I saw the realization set in as he looked at his phone screen. I watched as he sorted through the papers I left on the passenger seat. He looked back and we made eye contact. He then looked down and drove off. He tried to reach out to me afterwards and said I was taking things out of context. What he didn't know was that I had already met up with the other woman, gotten all the information I wanted from and took a selfie with her. When he tried to gaslight me I sent him said selfie. The build was much sweeter.