Having a pretty rough week and episode of OCD myself actually. Just got up to make my own post and came across yours. I thought I'd break it down for you a little. At least give you some helpful information that I know and has helped and comforted me in the past.
Okay so first thing is, those thoughts you are having are definitely OCD related. OCD can manifest in a few different ways, one of those ways is intrusive unwanted thoughts. The worst part about the intrusive thoughts factor is that most of the time our brain pulls the scariest thoughts we have....the ones that will inspire the most fear, the most emotion, and puts them to the forefront. In return this brings about mental chaos. I'll give you an example....there was a woman I read about before who basically was a loving devoted stay at home mom. She took care of her 2 kids all day while her husband worked, made their lunches for school, made their dinner, put them to the bed, the whole mommy routine. She absolutely loved and adored her kids, however, her OCD basically manifested itself in intrusive unwanted thoughts about her harming her kids. She would keep getting thoughts of her hurting them and thinking she might do something to harm them even though she of course knew she would never do such a thing.
That's just one example. So you see, it's not only you going through this. This happens to a lot of us. And it's not my first time hearing about the cheating fear, or the fear of not loving a boyfriend or girlfriend. That ones actually one of the more common ones to be honest. I've heard about that one from a few people over the past while. Dealing with OCD is difficult, especially for those of us that are younger. I'm only 23 and I find that maturity plays a huge role in how we deal with our issues, even OCD. Just remember that it's not your fault, and these are not your real feelings or thoughts. It's just your OCD. Even if you have to keep telling yourself that. I can tell you one thing for sure though, the more you are afraid of it, the more it's going to happen. You need to try your best to let go of the fear of cheating and loosing your boyfriend. Keep telling yourself that even if this happened you would be fine and would be OKAY. Once your brain can change this scenario to one that's less devastating and harmful it won't flag it as a problem or fear and it will pop up a lot less often. That's what I realize. But when it IS happening, just remember its your OCD. I think it's great that you have such a supportive boyfriend. My boyfriend is the same. He understands my OCD and is very supportive which is a HUGE added benefit that many people don't have. There are so many people out there who suffer in silence or alone. Be grateful for the things you have and just weather the storm as best you can. The good news is that most people have these thoughts and OCD spikes in episodes....they usually come at times of high anxiety or when you just have too much free time on your hands. I can almost gurantee you this will pass and blow over. Feel better !