Hello mathman,
Your feelings are so completely natural. I have so much sympathy for you. I would find that situation torture. I'm sorry I don't have advice for you on how to move on from that, but I only assume time is all that will help, however hard it is to believe.
I have similar fears. My partner moved out almost immediately. We always talked quite a lot about work, but she always carefully avoided mentioning male colleagues, even though I know she has several. Long ago, she showed signs of not being very loyal. I have no idea whether her leaving me was related to someone else. I even fear that her choice of area to live in is somehow connected to someone else.
She recently said things that were similar to what you describe: That she wants to go and just live alone and be independent. But it might be a lie.
I think we can only focus on this part of what you said, "another part says it doesn't matter and it would only cause more pain."
Ignorance is bliss, I suppose. Imagine how much it'd hurt to know everything. By the way, I would personally absolutely assume the worst in that situation, but that's just my pessimistic character. But human nature can be very hard to bear sometimes.
I completely understand this gnawing at you. Sometimes when I feel that way I practice thinking "so what?" about what I'm feeling. So if I'm feeling ferociously jealous and hateful and I'm dispairing, so what? I carry on feeling that way until eventually I just don't anymore. The body and mind will get tired of feeling a certain way eventually.
Perhaps it's worse if you try *not* to feel the way you feel naturally. Maybe trying to escape these terrible feelings will make them worse.