This is definitely an anxiety problem, and one I, too, suffered with.
For me there are a few things I worry about obsessively (health, uncertainty in the future and my DP cheating on me). They all follow the same patterns of thought - irrational thoughts, obsessive thoughts (i.e. thinking same things over and over) and unreasonable thoughts, so I would be CONVINCED despite all evidence.
I checked the bedsheets for stains once, and I (being a brunette) found some blonde hairs in the shower, too, probably from coming in on our clothes or from a friend or something. I used to obsess over my DP's going out clothes and even once inspected boxers for stains.... gah. I was terrified of my DP's bags in case I found condoms and was obsessive (ie going over stuff) and avoidant (NEVER looking at his phone etc even though he wasn't remotely cagey about it).
I go through phases of worrying about all the things I worry about, but I did manage the nip the cheating worries in the bud:
I explained to my DP that I was suffering from anxiety, related to uncertainty (as you have to TRUST), and, even though it REALLY irritated him at the time giving me the reassurance he actually promised me he had never and would never cheat on me, and he said it was the last time he would say it. I don't want to get you into the habit of seeking reassurance often, because that is also a kind of unhelpful obsessive behaviour, but I think having him look me in the eye and say that REALLY helped.
Then I wrote down that no matter what "evidence" I found, I wouldn't ask him about it. Because he didn't deserve it. I wrote down how I felt when he made his promise and I made myself promise that unless I found three different bits of corroborative evidence, or one big one that was unequivocal, I would never bring it up again.
I read a lot about CBT as well - have you looked into this? Basically, often people experience irrational thoughts. So, you might think:
there is a brown hair in the shower
DP is having an affair
The latter one is the "hot" thought. I still have to watch myself now, for example, I will think:
Dp has gone to the pub for the second night in a row!
Dp is selfish/doesnt love me
What you have to do is realise that the "hot" thought is an irrational conclusion. Imagine you put the "thought" on trial: what evidence is there for it. Is your DP the type to cheat? When would a blonde lady have come to your house? Why would she have showered?! Does your DP show you he loves you? Wouldn't he leave rather than cheat?
So you see from the above there is a lot of evidence AGAINST cheating and no evidence FOR. Then, you decide on a new thought, e.g. "there is a hair in the shower because life is not always explained and there is probably a blonde hair on my coat somewhere" or something. For example I once did find an extremely tiny and almost invisble stain on the bedsheet and my CBT "reasonable" thought was that it was from me or that it was permanently stained beyond washing etc... sure enough it's still there.
Do you see how I mean? Then once you have decided on your more reasonable thought, consider the matter done ans FORGET ABOUT IT. And each time your thoughts go there, do not repeat the above, just say to yourself "I dealt with that and I'm moving on."
It will get better.
xxxx