My perspective: Do not marry this man at this time.

The situation sounds complicated and might shake out in a number of different ways, but your relationship is on shaky grounds if he’s in love with this other person and spends a lot of time with her. You feel constantly criticised (not cool) and wonder what life might be like outside of the relationship.

I don’t think you should break up right now, although that’s an option, but kick your wedding plans into the long grass. If you’re going to stay together, there’s no rush and if not, divorce is a paperwork nightmare. Don’t do it right now. Talk, think about what you want, listen to him and to her about what they think, and go from there.