I've just returned from a very slow 17 miles and every revolution of the pedals was a struggle. No physical reason - near perfect weather conditions, a nice route and a great bike.
But for some reason I just couldn't switch on mentally. Bizarre. Every so often I struggle to motivate myself to go out, but when I actually get out I nearly always enjoy myself.
Not this evening though. Ah well, there's always tomorrow.
Just enjoy what you can.
My mental motivation is there, physical not. My back is fubar'ed. I took a good day last week, and did two hours plodding about on the MTB, quite hard stuff, but I spent a bit walking, where my bike was capable of doing the down hill. I was the slowest down a section as I walked, shat myself about crashing (broken spine in Nov). I was down the bottom on Strava on the descent, but compared to 40 something out of 800 riders out of a climb and I wasn't trying a little earlier...
Crap happens, and sometimes it's not a good day. I can't ride like I used to, but at least I am getting out a bit. It's just 10% what I was doing, but that little bit is actually better than the whole !
It's about enjoying yourself. I was a serious roadie, I have a tonne to learn about climbing a hill off road. You can't apply power like you do on a road, you have to ease off... WTF.. that's alien !!! I'm riding more off road now, but it's a huge learning curve.
Don't give up, just enjoy it.