I’m posting this here because I don’t feel like I have anybody close that can help.

So my boyfriend and I have been together for about a year and a half. We go to the same college and will be living on the same floor next year. Before we dated we were best friends in high school. He liked me for almost two years before we dated. So we began dating and everything was amazing (or so I thought). A week before our 1 year anniversary something told me to look through his phone. He had tinder and was texting this other girl. He’d been texting this girl for FOUR MONTHS. They’d never met or sent pictures. He said that he only texted her when he was horny (compared it to interactive porn). The thing is I didn’t find out until two months after they’d stopped talking. And the 4 months that it happened were the biggest of my life at the time (prom, graduation, and I’d got kicked out of my dad’s house). And he made those times better for me because I didn’t know what was going on. We’ve talked about it and I’ve dwelled on it for what feels like way too long (it’s been a couple months now) and I don’t know what to do. He’s my best friend in the entire world and every time I mention that I’m not okay he’s willing to talk about it and reassure me. I’ve told him I was debating breaking up and he cried about it and says that it skyrockets his anxiety and he can’t imagine a life without me. But I feel like I have to make a decision and stop dragging it out. What do I do?

EDIT: The worst part of finding out was how well it was hidden. Like I said, it was prom and graduation and happy times so there was no distinct change in behavior. We were together 24/7. We even worked at the same job.