I feel really silly about this (hence have changed nickname to raise it) but it is really a pain...
My first love gave me the boot when I was 18, after a year's very intense and really happy relationship - he was a year older, off on gap year etc and he said "it didn't feel right" to continue our relationship. We both went to university in different places and I got on with live as best I could, even though I was devastated and my confidence had been absolutely crushed, and in fact, made a point of trying to have the most interesting and adventurous time I could, as if to prove to myself that I wasn't such a no-hoper as I felt. I had a couple of relationships which were good but not as absorbing and am now with DH, who is great, and we have two amazing DCs.
I've seen my first boyfriend occasionally at social events (we have a couple of mutual friends still) over the years and we had pleasant friendly lunch together a couple of years ago, but we didn't stay in touch.
I didn't feel any kind of attraction to him, so it isn't a case of still fancying him/wanting an affair. not at all.
All the same, I have really strong intrusive thoughts about how things were when we were young, and how close we were, and keep thinking how great it would be if we could be good friends. this all started in earnest when I had DC1, even though I'd been thinking in these terms for a long time.
given that I have so many good things in my life, I'm really frustrated that I keep pondering on something in the past like this.