I don't think this is small or stupid at all. If he's cheated on you once before then he's broken your trust. Constantly texting this other girl and spending time at her house by himself without even calling you like he said he would does not seem to me like he's interested in earning your trust back. It's great that you came on here to rant rather than ambush him because that probably wouldn't have helped matters. But I do think a calm, collected conversation in which you lay out your completely VALID concerns is warranted. You may have to set up some strict boundaries such as him not seeing this girl anymore period unless you're with him, and then maybe he can see her by himself once you feel comfortable enough. Sounds harsh, but he cheated on you and he needs to suffer the consequences if he still wants to be with you.