How do you fix trust issues?

I always bring up my trust issues with my partner; he lost my trust a while ago. He asks me what can he do to make it right and what it will take for me to trust him again and I honestly don’t know how to answer that. We always sit down and talk about how to improve our relationship and I’m always dumbfound. I want us to make it and I really want to try and work on us but I don’t know how.