I genuinely don’t know why I was so stupid to keep believing it I really don’t I just thought he was honest and I trusted him
When you are an honest and trustworthy person yourself, it's actually very hard to wrap your head around the possibility that someone who says they care about you could be blatantly deceiving you with no apparent remorse.
You know that your own conscience/sense of right and wrong would not allow you to live that way without causing you extreme stress, so it's hard to grasp the two-facedness of someone who says they care about you and then does things they know are hurtful to you.
Don't feel stupid – trusting people to behave honestly and respectfully is normal because most people are like you; they simply couldn't or wouldn't treat others dishonestly or disrespectfully.
Occasionally though, you encounter someone who is not normal, and can and do behave like this. They get away with it for the very reason that most people don't behave like this so most people don't expect others to behave like this either, and they give the dishonest person the benefit of the doubt.
You've learned a tough but valuable lesson. When your gut starts screaming at you that something isn't right, it's a sign to listen to yourself, sit down, and look at what's going on with a different set of eyes on – not the eyes that love and trust this person, but the eyes of a detective or maybe your mother or a dear friend who cares about you. How would they see what's going on?
Write the situation down in detail. Think about what questions the objective person would ask. Read back over what you've written and try to pick out the elements that don't line up ... like the fact that he told you his ex won't speak to him, but her parents are apparently happy for him to visit regularly.
Apply your own logic: if you had a daughter who didn't want any contact with her ex, would you really invite him into your home on a regular basis? No, that's ridiculous. So something is clearly off, something that cannot be waved away with vague or convoluted explanations, or a declaration that you "should trust him, because he prides himself on his honesty." In fact, anyone telling you (rather than showing you) they are very honest is a red flag in the first place.