Mabel, I'm so sorry - I do know that you aren't married, that word just crept in, I suppose in place of relationship. Many apologies. I hope I didn't offend you.
It's an odd balance I suppose, because in some ways you're in something that is very much a marriage, but of course it is also categorically not that. My mother's relationship broke down after 20 years and that was one of the many hard things about it - that she was in effect going through a divorce without any of the 'normal' channels.
I do understand what you are saying about the OM leaving the country at the end of the year. Is it possible though that you are focusing on that as being the 'separator', when in fact it is your strong decision of last week that is the separator? If he lived just down the road with no intentions of going anywhere, the distance between you would be just the same, actually. As things stand now, you cannot go there. Ever. It makes no difference if he is two miles away or two thousand miles.
Nothing you have said sounds terrible. I have learned that you never know what is around the next bend in the road. You can't be sure of where you or OM will be in two/five/ten years time. Saying 'I'll never know what I missed' is just you hurting yourself - turning the knife as it were. Because none of us can read the future.
All you can do is make the best decision you can, based on the information you have at the time.
I think that there is a part of our makeup as human beings that sometimes deliberately causes us to hurt ourselves in certain situations. Think back to being a kid - surely everyone picked their scabs sometimes. It hurt like hell, but there was a sort of delight in it at the same time, somehow.
The feelings you got from OM were ones that made you feel wonderful, and therefore grieving over losing him, however odd it might sound, also contains those feelings. Saying to yourself 'I will never see him again' causes pain, but that pain keeps alive in your emotions the feeling of loving him.
The place you are aiming for is where you are 100% able to not need to relive those strong emotions. I think that all these things you are feeling and going through are normal stages. Yes, they are excruciatingly painful, even more so because they are allied with feelings of guilt.
But remember you have chosen this path because you are strong. It's not an easy one to walk down, but the best paths never are.
Personally, I think that while in healthy relationships an element of fantasy and imagination can creep in to sex occasionally, in your case it's not going to help you one bit to think about OM during sex. For exactly the reason I mentioned above - it's keep your feelings for him simmering away, however quietly, and therefore stopping you from moving forward or giving complete attention to the relationship with DP.
To expand on the (admittedly dubious) cooking metaphor, you can't keep OM on a low heat in a hidden corner of the hob. You have to remove the pan from the heat and wash it up and put it away.
Oh dear, peculiar images of men as saucepans now flit through my mind . . .
And in answer to your question, yes, I have been in a comparable if not precisely similar situation. And I echo ilovemyteddy's fantastic posts upthread. Proceeding any further than you did would have caused you and others far worse pain than you are feeling now, however impossible that may seem at this moment.
How was your evening? How are you feeling today?