Well...why does she trust him? Why is he trustworthy?
Show us that.
It's going to be a little different for everyone, really, as far as trusting someone romantically. There are the basic things like calling when he says he will, or arriving on time, but there are deeper issues, too.
My Downside books aren't romance (they're UF) but one of the things that leads the heroine to really trust the hero is how much he understands without being told, and how he reacts to things. He doesn't judge her. She also had an abusive childhood; at one point she shares a story about that with him, which is kind of a big thing for her (he already knows, because he just does/figured it out, but she's never really
discussed it with him--one of the reasons she trusts him enough to tell the story in the first place is because she knows he's figured it out and understands). And his response is basically just to say, "Shit, Chess," and not really much else (her name is Chess).
For some women that may be a sign that he doesn't care, but for her it's him just accepting it, accepting her, not pushing for more information or trying to make a big deal out of it or trying to get her to open up about her feelings or whatever. Something awful happened to her; he's clearly sorry for it but it doesn't change how he sees her or feels about her and he's not going to make her talk more about it. That makes her feel like it's okay for her to tell him anything in safety, and not worry that he'll see her differently or make her say or do anything she doesn't want to do. He lets her deal with telling him in her own way, basically, and doesn't try to make it about him or his feelings about it.
But again, that's her, because I'm sure there are women out there for whom "Shit, Chess" would be a cold or unfeeling response. I'm sure there are women out there who would expect him to say something more about it or try to tell her it's okay or whatever, but that would have been a bad response for her and would have made her feel more uncomfortable.
I hate to bring anecdotes from my work into threads here--and I'm certainly not saying my way is the best or only way or that I did some great thing--but do you see my point? It's really about what kind of woman your MC is and what she needs or wants from the hero to make her feel safe and cared about. What made Chess feel safe may make your MC feel totally unsafe and uncared about, and vice versa.
Does your heroine need him to listen and ask questions while she cries, or does she need him to change the subject with understanding on his face, or does she need him to get angry or upset when she's angry or upset, or does she need him to just keep calm, or what?
Once you know what makes a man trustworthy to her, it will probably be easier to make him trustworthy.
