I relate to this so much, wow. It sounds like you both still need to heal some things about the past, and it sounds like from what you say, he would be willing to do this with you.

An affirmation I used during my last relationship was, "I am safe and okay." I would dedicate 5 minutes a day to sitting, walking, or just being with this affirmation. To reaffirm to myself that I am safe now. After two weeks I noticed I wasn't as on edge or anxious, and felt more secure. I still use it when I need to and immediately feel calm and centered.

I don't have much advice beyond that, I have trust issues myself. And I haven't learned how to deal with them in healthy ways and resolve them yet. But I hope that helps you.