Hi, well that's not a comfortable situation to be in :/ You're still in love with your ex, and at the same time you know you can't do anything about it but accept it. I'd say that's a good first step to mourn that relation. Because yeah, that's what it is about really : mourning. Not your ex in himself, because hopefully he's not dead. But mourning your relation together, the future that maybe you planned with him (even if it's just two days ahead, I don't mean necessarily "a house, family and evening around the fireplace"^^). Because your future will be without him, and yeah you have a loss here, it's important to mourn it properly to be able to move on with your life.
It's like, allowing yourself to be sad, to take the time and give yourself the time to accept that your future will be different. And different doesn't mean less beautiful/exciting/happy etc. I know it can seem like that, that you won't find someone you'll love as much etc. I'd say that well, you won't love someone the same way. Not because you'll never that happy again. But because I believe that we love people differently each time, because they're different people each time.
What you lived with him, what your shared, they way your relation worked etc, it was due to the combination between the two of you. It will be different with other people, but not "less". It'll just be different. And maybe you don't want "different" now, because well, we like what we know, what we're used to, and change is frightening.
That's ok to be scared about what can come next, but in a way, if things didn't work out with him, that mean that something else is possible, right ? And that change is actually a good thing (to, eventually, find someone with who the relation will work better).
Some people would say : go on dates, find other people to "forget" your ex etc. And well, maybe that work sometimes.. But I genuinely think that allowing yourself to mourn your expectations with your ex is the best way to move on. The point is not to forget, I think. We can't just "forget" what hurt us... The best way to not feel pain anymore is to process things, not try to bury them ^^
What you lived taught you things. I don't know how your relationship was, but even if it was bad, abusive or things like that, it can be of some help for your to know your needs and what you want (and don't want) in your future relationships... to understand yourself better etc. So they're all teaching moments for us to be who we are, right ?
I'm not sure my answer is what you're looking for, but I'd be glad to talk about it with you and be your listener :)
Take care, you're on a new journey, lots of things to look forward :)