I was in your shoes 3 years ago.
24 years together, the last decade of which had been awful. I put on 7 stone from comfort eating. It took me a long time to see that it was over, and why, and once I did I knew it would have to be me who ended it because it always had to be me who did everything.
It's a long time to be with someone and your lives are very intertwined after that kind of duration. You can probably hardly remember what 'before' was like. So yes, very normal to be upset and scared, even when you know what you're doing is the right thing.
In the time since I ended things, I've been on my own, not looking for a new relationship, reacquainting myself with the real me who had somehow got lost in all the mess and sadness, doing therapy, making a new, more grown-up relationship with my children, finding my anger about all the stuff that had gone on between me and the ex (I put up with a lot, far, far more than I should have), dealing with a lot of physical and mental health fall-out, getting my shit together professionally and financially. It's been really, really hard but so worth it.
You will be OK. I promise it. It will be tough but you will come out of this stronger and happier.