I can so relate to you OP. Me and a friend nearly got together 20 years ago but I had just started a relationship with someone else and thought it wouldn't be fair on that person not to see where it went. It's a choice I've often pondered over. Anyway, me and this guy went away for a couple of years to see if we were right for each other, when we came back, crush was with someone, they now have two kids in their teens. I broke up with this other guy, was single for a long time, but 10 years ago met someone and we now have one DC.
I've seen him here and there over the years and last year saw him at a party where we had a longish conversation followed by an evening of flirting with him basically telling me how he felt about me. After that we saw each other once a few months later. There hasn't been any direct contact but I know he's thinking about me and me him.
I went through hell, considering my own relationship, considering being on my own, considering everything. He was my every waking thought, first thing in the morning till last thing at night. Obsession doesn't even come close. I came to the conclusion that I was very comfortable with DP and had no guarantees that a relationship with him would be better or worse. I thought I'd rather be with DP than be on my own. I still don't feel 100% committed to DP, basically, I'd love a stolen night with this other guy just to see what would happen and where it would take us but I know that it's impossible at the moment so I try to ignore it.
The horrendous crush feelings lasted from when we talked at the party right the way through to recently, so roughly 6 months. Now they have subsided somewhat - I feel like we've both realised it's impossible, the lives we would hurt would just be too much to bear. But unlike other crushes I've had (much shorter lived), the feelings haven't gone completely, they're just manageable and less obsessed.
So, in short, it will pass, hang on in there, don't do anything you might regret, don't have direct contact of any kind. The madness feeling will fade, even if it seems to take forever.