Relationship OCD

I’ve never been one to have anxiety or anything before but have known about OCD for a while. I woke up about 6 weeks ago having a panic attack about my health (I have a baby boy always worry about dying) this then turned into relationship anxiety. Me and my partner have had a baby, we have a mortgage together and he’s recently proposed to me. But I still can’t budge the fear of him cheating on. He’s constantly reassuring me and I’m convincing myself something isn’t right. He has a very strong opinion on cheating, the same as me. We can’t stand it. My mind is playing so many tricks on me, I’m constantly tracking is location and checking the ring doorbell.. I’m noticing things that have probably always happened such as him taking his phone into the kitchen while he makes the baby a bottle instead of leaving it on the sofa. It’s driving me insane and it got to the point where I convinced myself he was sleeping with the neighbour and then my best friend. He’s been cheated on in the past and so have I. I fully trusted him then all of a sudden (he didn’t do anything to make me feel this way) my brain is convincing me he’s cheating on me. He has 2 other children and literally says he wants them to look up to him and that he loves me so much he couldn’t even look at another woman in a certain way and he loves my family so much basically explaining that there is so much to lose for what? But all of this doesn’t seem to help me. It’s like my brain won’t chill out until I’ve found that he’s cheated on me but I’m not going to find that. I then started worrying at how do I know what he’s saying is true and he isn’t just saying it to make me believe he isn’t a cheat and that’s when he and other people have said you just have to trust what he’s saying. My argument is a cheat would say they aren’t a cheat. He’s said if he ever did (he explained he would never) he wouldn’t be able to live with himself, he’d have to tell me as he couldn’t cope with the guilt and also having to hide his phone but my point is he sometimes has to work away with work and what’s stopping him telling someone a completely different name and then never being able to contact him? I’m also worrying that what if he picks another girl up on the way to his hotel when he works away and when I ring him to speak she is just quite (because some girls really would do this) & then when I FaceTime him in the room she just waits outside. I just don’t know how to budge this feeling or what I can do to help????