I guess I used to have those kind of thoughts and I suppose the more involved you are in a relationship the more prone you are to having them. In my case, they were justified as ex was lying and most likely cheating and if not then the lying and involvement with multiple OW even on a psychological basis was ridiculous, coupled with how he treated me. So all things combined where I thought it was healthier to leave the relationship - and it was
My last boyfriend I didn't have those thoughts because honestly, he was a bit below my criteria and I figured if he found another woman who he was into and who was into him that would not crush me. I could deal with it.
Now I'm involved with someone and he travels and it's a relatively new relationship and I know him well enough to know that he's had some past issues with relationships and wouldn't do that to me, in all honestly I think that he probably struggles more with me being faithful than I do with him, or it's an acknowledged fact of life right now that new relationship maybe we just shouldn't be worrying about this stuff ahead of the game, so to speak. I would rather be shocked by finding something out than run myself ragged by worrying about it. Really, I think some perspective is in order in a balanced relationship, the worry and reassurances are bilateral. In your case you are separated so presumably not meeting each other's sexual needs, and is there really an OW or is it imagined for you too? My inclination is to not be too challenged by contenders. If there were serious contenders my guy would be married, and would not have said the very nice things he did to me. He's way too busy to be a player at that level, lol.
I think maybe lack of confidence and not understanding guy's position when it comes to jealousy makes us feel like we worry about the guy but we think that because we feel honest and we know we would never cheat, it should be obvious to the guy. It's not. And if they are cheating or thinking about cheating, you can be sure they're spending some significant time worrying about what you would be doing when they're doing that!
In the end you should not spend your energy feeding the beast so to speak. Jealousy is one thing but if you let it possess you, you will go nuts. Then because you're nuts, you'll be thinking, well, why wouldn't he cheat on me, I'm a wreck!
So, as you can see, don't go there until and unless you have proof. Focus on your securities in life, such as that you are loveable, you have pride in being honest, you allow yourself to love someone (and in the process of that, try to address your worries and spare them from having them expressed inappropriately, i.e. out of line comments...) and find ways to get yourself reassurances that you are on the right path.
You can always read Othello or The Winter's Tale (or watch movie versions) to get a sense of the importance of getting a grip, so to speak. Focus on ways you can make yourself mentally stronger, and ways to be emotionally stronger, and on ways to make your relationship more emotionally intimate. If you keep focusing on the physical, you're not going to have the security you need to go the distance.