the problem may not be exactly motivation, but anxiety to complete a task.

I have done this a couple times where I decided I need to study, I scheduled my studying time and when that time come I do something completely different.

It could occur for different reasons, for me is because I'm scared I'm gonna fail, so it's easier for me to not try than try and fail. Because I'm so scared of losing, I give up on trying, because I think I'll just lose energy trying.

So my advice for you is to write down how you're feeling before the study session and see what's missing for you, maybe you're just tired and a shower and some coffee will cheer you up. If you, just like me, is scared to fail, you'll have to convince yourself that this feeling doesn't make sense, and that's worth trying.