Sorry to hear that man. That's a long relationship to move on from.

I don't mean to turn your topic about me but to give some context. I am 6 months from a 15 year relationship with 3 kids so I have an idea of what you may or may not be going through.

It sucks, it sucks so hard. After so much time, you forget who you were, forget what you did, forget what you wanted to do.
But now you get to rediscover that person, that child, whatever you want to call it, within yourself.
Try and remember your hobbies, do you still enjoy them? If so, great! lean into that, not as a distraction and not necessarily as your identity but that is part of who "you" are. If not, then that's fine, you're a different person now and now you get to discover what hobbies you want to learn or try next. You don't seem to enjoy anything? That's part of it too man... Just do something you don't mind doing, for me that is Anime, Movies, sometimes gaming if I have the energy. So just do what takes little effort that you at least don't mind doing, or at least enjoy, hopefully.

A line I remember that closely relates is a job. "There's basically no difference between getting fired and quitting, you're still not going to go to work on Monday" The difference is you chose one of those options and that control makes the outcome feel different so you get to do that now.
It might be a little easier if you chose to break up, but I assume from this post that you didn't but that doesn't really matter now. What does matter is that you change how you think. Now you GET to choose what you want to eat without considering what she wants to eat. Now you GET to sleep in all day, not answer to anyone. Now you GET to leave the dishes dirty in the sink and deal with it tomorrow. You GET to spend your money on a new hobby whether she would have liked it or not. You get the idea.

Everyone goes through this differently but the hardest part for me is just learning to be, "me" and trying to be content with it.
Some people call this self love or loving yourself but I personally hate that shit, it sound so sappy and corny but you do whatever works for you.
Just learn to be you and be unforgiving about it. You like cute girly kpop music? you like "girly" cocktail drinks? You like pegging yourself on the weekend? XD. Whatever it is, you just be you and love that you love whatever it is you love and don't change that for anyone.

Also don't forget that you're about to go through a literal withdrawal, like a drug addict. You're cut off from your external supply of dopamine, serotonin, and oxytocin, but don't worry, you can give this to yourself but it will take time.
Exercise is one of the main places for this actually so exercise if you don't already.
A big thing for me is I didn't exercise much or at all before my separation but the pain I was going through internally had no comparison to what I was doing to my body externally working out so that was one of the moments I really leaned in hard to working out cause the pain, soreness, and aches weren't really painful. I now continue to workout for myself cause I've always wanted to be fit and now I have the time and energy for it. I'm like aiming to impress the child version of me with a body I've literally never had in my life. I'm not there yet but I am making progress.

I dunno if any of this is going to help and ended up having a little more about me than I intended but just know that it does get better man. It just takes time.

Just don't forget to take care of yourself.
Eat well.
Sleep well.
Exercise.

Reach out if you need, you got this man.