I'm so sorry OP, you sound really down, I think maybe this has just brought those feelings to the fore.
I think maybe harbouring these fantasies about someone unattainable was a sort of escapism from your unhappy marriage.
A lot of us do similar things, it's no different to thinking "one day I'll win the lottery then life will be great", or "We'll move to a different town and things will be better", or any of the fantasies we all secretly have that one day something will change and life will be wonderful.
It's just your mind's way of coping with the drudge, if we're unhappy we have to think that it won't be forever.
This ex isn't the one, he never was. Perhaps this will be a good thing in the long run if it makes you re-asses your relationship with your DH. Whatever action you take is of course up to you but perhaps this will prompt the change you both need.
This feeling will pass, just try and use it to start making a positive change.
(It could be worse, you could have just found out that the lovely college boyfriend you left for your oh-so-wonderful new man 12 years ago got married last week. And you could find this out mere days after the anniversary of separating from said oh-so-wonderful man because he cheated on you repeatedly. Now there's a festive slap in the face with a wet kipper!)