I'm trying to write canon/lore for a series, but haven't been feeling motivated to do so for some time now. How do I get out of this slump?

Keep writing, but start writing something else. Find an online story prompt that interests you and write about that. Write about a real life incident that happened to you and how you felt about it. Write a movie or book review. Just keep writing (and reading books) to improve your ways of telling a story.

Simple
Quit wasting your time on worthless crap, daydreaming you're a successful author and go write instead.

You're playing pretend, by pretending you're working on a novel, when the reality is, you're wasting time on an elaborate fantasy world, which will likely never be used, unless you did have a best selling series, in which case you'd be able to trot out all these sequels.

You know what? The silmarillion sucks balls.
There I said it. It is bunch of worthless myths which you only read because Lord of the Rings was so great, that you pathetically clutch at anything to remind you of how great it was.

You now what a real author does?
They keep throwing stuff at the wall till something sells, instead of mooning over their private fantasy world and dreaming of being successful.

Only time will tell.
Sorry, but there is no simple answer to this one.

just reward yourself after you do it

Well, there are a few things you can try. Meditation with relaxing music to calm your mind. Think about your stoey as you're falling asleep and let your partially lucid state assist with your imagination. If that doesn't work, just get a notebook and pencil and start writing. It doesn't matter what, just write, even if it is nonsense. Sometimes, the best solution is to just force yourself to do the thing you're having so much trouble with. It will stimulate the creative part of your brain and get those creative juices flowing.