Until a couple years ago, I always had issues trusting my significant other. No matter how great they were, my distrust of people in general bled over into my relationship. I was never obsessive or over-the-top, but it definitely caused small anxieties and annoyances. I think the main reason it went away was age, marriage, experience, and finding someone I'm comfortable with. Additionally though, without getting too deep into relationship advice, I realized the following:
1. Distrust is not about the other person. Feeling distrustful, though related
to the other person, has mostly to do with you. Figure it
out. For me, it was a low expectation of people in general-- my worldview.
2. My world view improved when I realized that if I could trust myself, the world couldn't be such a bad place. Even if I was the only one trying to live by certain standards, I count as someone. If I was living up to a certain moral standard, certainly others are too. Even if every person I know turns out to be a terrible person, it's a small sample. It sounds almost crazy putting it in writing, but internalizing this realization was a big moment for me-- bigger than just the issue I'm writing about here.
3. Unfaithfulness and falsity in itself is not the root problem for most people. If someone is acting falsely, there is probably a more serious issues to worry about than the act of cheating or lying, itself. Work to prevent these issues and if something does happen, use it as an opportunity to identify root problems or acquire new freedom. Value yourself over your relationship.