any advice at all?

I am finally in a relationship after months of dating someone. They have opened up to me about being cheated on in two of their prior relationships. I can’t imagine how hard it would be to trust someone after that. They say it doesn’t effect them but I can tell they are very scared of me leaving them. I try my best to see from their perspective, but I haven’t experienced the things they have. If any of you have similar experiences could you give me advice? They are very closed off when it comes to their emotions. Are there things you think I shouldn’t / should do? Things that might make them upset or happy? even reassured? I haven’t been in a relationship in a very long time so I would appreciate any POVs or advice at all!!