Not sure what to do

So found out my fiancé of 6 years fucked a prostitute because of what I did with his best friend which was having a 3some (fiancé,friend and I). Which was over a year ago and he did this a few days ago which I kind of don’t understand why until now. Anyways he says he feels guilty and that he shouldn’t have done that and he regrets it. I don’t even know what to do. I am an open minded person, I didn’t get mad at him. Just not sure how to feel or what to do. I have been thinking of just taking a break for a few days but other then that I’m torn.

He suffers from depression and is prone to hurt himself. This isn’t the first time I’ve left him. But I have been stupid to return to him. I still don’t know what to do cause we have so many things together.