It's a bereavement my love, no more no less.
You need to grieve for the past and the future.
You have to go through many stages, and they are not in order, some are easier than others, some are horrendous and some repeat, and hit you again out of the blue.
BUT you will, I promise, get through this.
I was married twenty years, three teenagers, ex had been seeing OW two years.
I was you five years ago.
What helped me was leaning on a few people in RL. Going NC with ex, unless it was urgent about kids, even then very brief and practical.
Most of all accepting that I would feel lousy, experience a wide range of emotions, and not denying them. Not wallowing, but going through them.
Treat yourself as you would your best friend.
You will get through it, you will find happiness again.
Think DD checking in with GP is a good idea.
And I also filed for divorce within 48 hours. This gave me control, and forced me to deal with practical stuff, but that's by the by.
Keep posting, so many of us understand x