Yeah, that's a tricky one. I feel like if it's something you're doing for free, then you have to enjoy it on some level. If it's not fun at all, then I'm not sure what you gain from "making" yourself doing it. You know? Do you normally work on novels, or on short stories, or what? Maybe if you threw everything aside for a while and wrote in a different format/genre, or even just did a week of a different flash fiction each day, if that might shake the cobwebs off? Just spit-balling here.