Stopping these overwhelming thoughts

[Kw94xx]

I'm not sure if this classes as OCD but I'm not sure where else to post. I feel like I have relationship ocd in regards to being cheated on. Every guy I have ever been with I've been messed about and cheated on and really hurt me. Now I am in a relationship with a guy who I really love to bits and I have been with him for a year and a half. For the past few months I have been having awful thoughts that he has cheated on me at the beginning of our relationship even though he completely denies it. I have no proof but I have once found out that he had messaged a girl calling her "fit" and saying they should meet up right at the start but I found this out not too long ago. However at the same time this girl was messaging him at the start, he said that a girl he slept with before was trying to talk to him but he told me back then that he set her straight and said he had a girl friend. But now I know he spoke to that other girl how am I supposed to believe that he told a girl he has a bit of a past with that he wasn't interested? I also saw at this time that he liked her photos of her on Facebook with barely anything on so I said to him then why would you do that if you say you don't want to talk to her? I keep thinking about it despite him saying he didn't say anything to her and he hasn't done anything to hurt me etc. But it kills me that I don't know when I think about it it's like he could of slept with her in the worst case scenario or he's hiding what he's put to her or what! Or hopefully he's done nothing. I don't wanna keep loving in the past it's like I don't think he'd cheat on me or do Ote now because our relationship has progressed but I'm scared he's made a mistake at the start he won't admit to :(. How do I move on from it and forget? The thought of him being with or talking to someone else like that makes me feel sick. I have depression and anxiety any way and this doesn't help how I'm feeling. Do I let it go? If so how???