First things first, you need to look after yourself. Your MH is important and you need to prioritise that above everything.
Tell your fiance (by text if it's easier) that you're not necessarily breaking it off but that you need space and time to deal with this. If he loves you he will respect that and leave you in peace. Meanwhile, you are aware you will see each other at work and hope you can achieve a polite professional relationship.
If his remorse is genuine, he will do this. If he tries to pressure you in any way, that should tell you something.
If you think you need ADs or counselling, get to the DR and arrange it. See as much of your friends and rant away about him, post on MN, basically weep and rage and get it out of your system. RL support will be what gets you through this, and that is what friends are for.
If you already a lone parent and you're not living together, then you'll know you can cope without him both financially and practically. The emotional hurt will take time to heal, but it will.
No one can tell you what to do. If it were me, I wouldn't give a second chance for something like this, but I am not you. If you decide to make a go of it, you need this time apart in order to make it clear to him what he risks losing if he ever behaves like that again.