Cant stop ruminating

Perhaps triggering to ROCD This is gonna be long. Ive been with my fiancé 9+ years now, and i feel that we have a very good and healthy relationship 99% of the time. Of course we are still both growing together and trying to improve communication all the time as couples do. I know that we both love and care for each other alot, we plan to spend our lives together. During our relationship we have had arguments mostly revolving around my ocd and his own behaviors stemming from trauma and mental health issues that need addressing but we always pull through and come to an understanding and decide that what we spoke about made us stronger in the long run. Welll ive been dealing with ocd fears for awhile now about him cheating or lying to me or about there being someone else. Its been a couple years now since the theme started and i hate it. It flares every period and It makes me paranoid, no answer is good enough to satisfy for more than a few mins. I try my best to keep this to myself as it genuinely affects him and he feels that i dont trust him because of this theme. He struggles to get that intrusive thoughts are not thoughts i am actively choosing to think. I try proving to him that i trust him and I NEVER try to do anything to invade his privacy as i have seen upclose how toxic and unhealthy that can be for relationships. I try my best to believe in him, but my ocd always seems to pop up and ask those “what if,” questions i have to sit it it and its so unpleasant. Anyway he finds this theme very triggering because previous relationships he was in were volatile and full of constant accusations of infidelity and tons of insecurity. It makes him feel threatened and sometimes he reacts like ive accused him of something. I want to 100% believe he is honest and loyal as i dont think i could be with someone who isnt ever again, but ocd just doesnt work that way. Today i woke up and went into the kitchen and he panic closed out an app and then seemed defensive and said, “ yes i redownloaded instagram, you’re on there all the time” and opened it back up. I hadnt said anything at all yet this started an argument mostly on his side (my short term memory for what is said in these moments is not great especially when im stressed so i can only give the gist of what happened.) i was deeply offended and felt that it was a red flag how upset and defensive he got when i hadnt said anything to him at all, and mostly ive been trying to keep any cheating ocd thoughts to myself. He started justifying it without me even saying anything about it, all i said was “i only just woke up and youre acting like youre guilty of something when i hadnt even said anything.” And he said he knew i was thinking something. I said that i hadnt been but now im not so sure with that reaction. He said maybe you need to go through my phone (for reference i never have and never wanted to) with how things escalated so fast though i told him maybe i should so we can just let this go and he got upset and thought this was it that the relationship might be over. Of course this only made me feel more worried that he had actually done something. I told him i wanted to only go through it with him so we could move on from this. He tried to avoid the situation and have me look through it alone. I didnt want that i felt uncomfortable and made him come with me as he has a tendency to try and shut down and avoid conflict but sometimes these things need to be talked out. I found nothing and i figured there wouldnt be anything but after that reactivity out of no where i wouldnt have been able to stop having ocd either way. And ive been cheated on by previous partners my biggest fear is being deceived and being so naive and unaware and taken advantage of because im pretty ready to see only good in people, and pretty quick to tell myself its just my anxiety or ocd. I love my fiance so much and i think he is the most important person in my life. He felt relief when he realized i could calmly sit with him in that moment and that it didnt mean we were over he said maybe it was necessary to move past what happened and he thinks maybe he needs to go to therapy for always assuming that people are thinking the worst of him ( we have had moments like this before, just not this bad.) Unfortunately i knew that once things were improved and i was alone my ocd would start to ruminate and try to think of ways that something is still being hidden and that im being naive. I just want the rumination to stop i feel sick in my stomach. I just wish the whole incident never happened and i never ever wanted to look through something of his as i strongly feel against things like that. We always end arguments with reminding eachother that we plan to be together for life and that we love each other so i should feel relief, but my brain keeps replaying the negative parts and making knots in my stomach. Its rare when i see other people talking about the ocd fears of being cheated on so i dont know who i can talk to that relates most people without ocd just make it seem like i should “trust my gut” but that doesnt serve me with ocd i feel i cant actually trust it.