As already said, nearly always happens post LTR...

Especially if you are still trying to get over your ex...

After one of my breakups, I met up with a girl that I'd fucked before, and she was definitely a 9 or 10, one of the nicest girls I've been with....
She is ready and wanting to mess around, and I just don't feel it...
Almost started fucking crying man! I didn't even know what was wrong with me...
I had hot pure filth on a plate and I just couldn't do it...
Told her I'm still in heartbreak and she understood and I fucked her another time so all good...

Don't worry about it
Also, maybe alcohol was the problem?