Hey, I lurk a lot and don't post much, but felt really compelled to reply to you.
This is my issue too. I had a fairly traumatic childhood and spent a lot of time in my 20s unconsciously seeking out men who would cheat on me and reinforce my idea that I'm not loveable and don't deserve to be treated well. I've had a lot of therapy and this is still ongoing.
I've been with my boyfriend for 9 months. Every time I notice he's liked a picture of an attractive girl on Instagram my heart sinks. It's a hugely physical disproportionate reaction and it hurts. I'm really lucky that my boyfriend is the kindest, most understanding man I've been with and I've talked to him about this at length.
When we first got together he'd been single for 2 years and he was still very much in a 'single' mindset with his behaviour online, it didn't even occur to him that what he was doing might cause any sort of issue for me because it's not about me. He was following and liking lots of hot girls on Instagram because he'd got into a habit of doing it: he was single so why not! When I saw he liked a nearly naked picture of a girl he knows in real life I asked him about it. He said he did it because she clearly needs the validation and there was no 'intent' to it, that it was one of many pictures he was liking as he was scrolling. I explained that she might need the validation but in him giving it to her it took something away from me. He understood that.
When I'm feeling shitty about myself I end up looking through what he's doing online. I have a mental list of girls I think are attractive on his FB and Instagram and I go through their pictures to see what he's liked / commented on. It's insane. It's invasive and insane. I struggle really really hard not to do it. Sometimes I do and I hate myself for invading his privacy.
Sometimes I might ask him who a particular girl is and he'll tell me. We're really open and honest about everything and he understands that I'm working on this issue and that sometimes I need reassurance from him. However, every time I do it I know I damage the trust between us a bit so I try hard to reassure myself.
There's a world of difference between what your boyfriend is doing and cheating, but maybe you need to do what I did and have a conversation about online boundaries. Do you consider him messaging girls emotionally to be cheating? Sexually? I do. Does he? What about following porn stars? What about cam girls? Decide what you think is acceptable and see where he stands on things - put those boundaries in place so you can relax knowing what you both think is acceptable online behaviour. Offline behaviour is more obvious, I think!
With your own behaviour, perhaps you are doing what I do and are looking for something preemptive that could suggest he will cheat. Given my history it's understandable I do this. But when I think about it long and hard liking pictures isn't ever going to tell me if he's going to cheat on me or not. I could like thousands of photos of men - some of whom he might be worried about - but it doesn't say if I have any intent there or not. It's just liking pictures. I'd rather he didn't do it, but then I think about all my male friends who like photos of me online and their partners. They're all in (I assume) happy relationships and just because they're liking my pics it doesn't mean they want to sleep with me or cheat on their partners with me. At least, I hope not!!
Ultimately you need to have a conversation with him, especially if this is going to cause you problems. Put your boundaries in place and learn to let go and trust. You can't control what he does online and you need to trust that whatever he is doing is not being done to hurt you.
Unless, of course, he's a cheater...! But try to come at this from a place of trust x