Starting over after cheating?

My boyfriend and I have been together for just over 2 years. He moved to university and joined a group chat where he met this girl, he got pretty close with her but told me she's nobody and he's just helping her with some work she had to do. I always had a very off feeling about their friendship, and at the beginning of December he told me they had kissed on multiple occasions. He told me he's depressed and he feels really detached from his body so that's why he keeps doing stupid things and he said it's not helping that I'm not there with him all the time, I really feel like he's being genuine about this because I knew something wasn't right with him and he had told me he wasn't happy but obviously I didn't know it was this bad. We agreed to do a completely clean slate and he's cut off contact completely with the other girl, I've been trying really hard to forgive and forget but I just keep having anxious feelings all the time about it. It's a really difficult situation because I want to trust him again and like I said I feel like he's being genuine with me that it was a big mistake, but I just keep thinking about it.

Any help on how to move on from this?