I sometimes get the same feeling, especially after a period of inactivity. Not sure why but possibly related to my occasional bouts of depression.
I can't offer you specific advice as everyone is different, but what works for me is to "force" myself to ride on nice sunny days on some of my favourite rides, with no targets or timescales, and I realise how much better I feel for it and how much better it is than the alternative of doing nothing. It often takes a few rides before I get motivation back.

Good luck.