My partner is very understanding luckily. He's got used to going off an enjoying himself without the expectation of me joining him.
I've given up accepting invitations as i never know how i'm going to feel on any given day.
It's quite annoying not being able to plan anything. After a while you get used to it, and used to letting people down!
I've come to the conclusion that those friends that are still around despite this are true friends and those that fall off the radar i'm better off without!