Asklemmy

My partner of just under 15 years left me out of the blue three years ago.

It... wasn't easy. I understood her reasons, but that didn't make it emotionally more acceptable, especially having not known about it beforehand as after so many years it was the only big thing, "never keep anything back at all, no matter what".

But I suspect my case was a bit different than yours, as the problem wasn't the first night. She had been gone for a week or so for work before, or even 2 weeks for a Mallorca trip with a bunch of friends, so the first 3-4 days were perfectly fine. It hit really hard after about a week, and then lasted for about a month.
What I had to do was significantly change the flat. Not only rip out stuff that was "hers", but more importantly reorganize rooms so that the association was gone. It helped. Once that part was changed, I could pretty quickly calm down. I needed that constant reminder of "us" gone. As if I had just moved into a new flat on my own, and this was all me!

That being said, there was one other thing. It sounds vain as hell, but I got a fuckbuddy. Sex helps. Happy hormons and all. It didn't help on its own, but coupled with steadily changing the flat around me and engaging more with my personal hobbies over our shared hobbies, it was enough to get me through it.