I know the "I just don't feel like it" block. Although, at the moment, a bigger one for me is "I'm just too busy".
My most effective way to get past both of them is to have something prepared to work on that I can do in a really short space of time. Just 5 minutes. Or even 1 minute. It could be an exercise that you're trying to get better at. Or just playing once through the chords to a song you like.
Whatever it is, your task is just to play that for a couple of minutes. Once that's done, you're free to stop and do something else. Most of the time, though, I find that the motivation has appeared and I stick with it. If not, then that's still fine. I have no need to feel guilty because I've done my bit of playing - short though it was.