dear dreams:
I am going through the exact same thing, with the exception that I am already married to him. Same thing. He calls it a big mistake, doesn't know why he did it, tells me he is willing to do what it takes to get trust back. Obviously regrets what he has done. Counselling is an option. We are in counselling. Even with all the right things, there are no guarantees. I have all the right things and am still conflicted. He says, does, feels all the right things. The question is do I? And that is what will need to make your decision. It isn't what someone else thinks you should be doing. It's what you think you should be doing, and when you're in a BAD place, like feeling betrayed, it's not easy to make that decision.
My plan is to behave my way to success, even if I don't feel it in the moment, the day sometimes the week. Deciding whether to stay or go, can also be a process. Let it be one if you need to. If he's in a hurry to have you decide to stay or go, and you can't hurry this decision, it's obvious he's not for you. If he truly wants to fix it, he'll know he can't hurry you.
all my best to you.