Just continue to try, and take it a day at a time. It sounds like you've been through some tough times and your reactions are based on your experiences, so your hesitancy is totally legit. Have you thought about processing your experiences with a professional to get healing?
I think being honest with your partner helps too. Letting him know how amazing you think he is but that you struggle with trust sometimes, so extra reassurances from him help. And be kind to yourself. You got this girl.