Have only come to this thread today, have read all of the OP's posts but only the first few responses. OP, I hope I don't come across as mean, I truly mean this kindly.
"And no, when he was single and I was in a relationship I wasn't willing to just break up with the guy I was then seeing to go see him on a whim, even though i was still in love with him, so perhaps this is a massive over reaction to his impending wedding."
Of everything you have posted, I found this perhaps the most telling. When you could have tried to have a relationship with this man, you CHOSE not to.
There must have been something behind that choice. I wonder if you have used this man as an emotional shield; i.e. you can't possibly let yourself get close to another man because they are not this man, the one you have decided is the love of your life. And you chose not to try for a relationship with him either.
His marrying has ripped that emotional shield away from you, and that is why you are feeling so vulnerable now. You can no longer tell yourself that there's no point getting close to X, because you and Y will "by some stroke of magic end up together."
Try to see this as a good thing because now, you can actually have a proper relationship; not fend the very idea off with 'I'm just passing the time waiting for him'. You only have one life, and you need to stop treading water.
Please put aside the notion that you love him - you don't. But you do love what this 'unrequited love' has given you. I'm not sure what that is; it could be space from other people. It could be safety from being hurt. It could be lots of things. That's for you to explore, and I truly hope that you do.
Best wishes.