It's a long one, sorry!
About 6 years ago I had an on again off again relationship with this guy. I had been severely hurt in past relationships so had big trust issues and never fully committed to being his 'girlfriend' even though he really wanted us to be together. It was an open relationship (my decision, not his) as I couldn't commit. In the end it got too much for us both and we decided to call it a day. He got a girlfriend about a month or two later which was very painful for me to see and we never spoke again as his girlfriend wasn't comfortable with us hanging out (completely understandable).
Since then i've had other relationships (both flings and long-term) and I am currently in a LTR. I haven't really thought about the ex in a romantic way since then - until recently. I was looking through old messages and emails with friends when I came across conversations I had with him. I had completely forgotten how sweet and patient he was, and how mature and loving the relationship was compared to ones I have had since. I'm sure it's me looking through rose-tinted glasses, but it made me feel incredibly sad and full of regret. I am a much stronger person now, and feel if I met him now and was single i'd 100% go for a relationship with him.
I don't really have any organic way of seeing him/talking to him, plus I love my current partner so much, so doubt anything would happen - i'm not sure I would even want anything to happen. But I can't stop thinking about what life might be like if I had bitten the bullet and gone for it. I have been fine for 6 years but now feel heartbroken all over again.
So is there any way I can stop feeling this way and thinking about him? With other exs i've just remembered the reasons why we ended, but with this one it was purely because I wasn't ready at the time, so it's nothing he did wrong. I can't stop fantasising about whether we'd still be together now and what we'd be doing/checking up on him to see what he's doing/feeling sad that he's not in my life anymore. How do I stop thinking about him?