Selfish guilt or delayed greif?

This all probably seems selfish and pointless in comparison to what others have experienced but I have found myself grieving more and more lately for a relationship I lost through my own doing, over 20 years ago. I know this is weird but there seems to be nothing I can do about it.

I am now happily married and have two smashing children of whom I am very proud. I am financially comfortable despite the economic situation and I have a very satisfying and challenging job.

Despite all this, I have found myself longing to contact someone I had an 18 month relationship with, over 20 years ago. I have no right to feel this way as I was the one who ended that relationship. It was not clear why back then, and time has blurred any reason for doing so. There was no one else involved. I find myself missing her so much on many levels.

I am not stupid and understand that there is no reason why this person would give me a second thought nowadays, but I really am grieving over the hole in my life that seems to be opening up for no reason. It is a real feeling of hopelessness which I find I have to give in to and let pass ..like sleeping through fog...rejection-like grief.

The worst thing is that when I get down, I cannot discuss my depresssed state with anyone as I don’t understand the reasons or the implications for this. There is nothing missing in my life that I should need to replace, and I don't fanticise over rekindling something that has been gone for such a long time, but I have lost the thread of why I ended the relationship and can’t escape from the feelings of loss and guilt…Mostly guilt.
I should be contented at this time in my life, but I can’t seem to let this go now for reasons I don’t understand...and it is getting worse.