There's an old saying, or whatever, that's been said about anti-depressants amongst creatives. "I don't want to be on medication. What if I lose my spark?" As though a little voice inside your head saying things like, "this isn't worth it, you suck, you can't write" is a spark I ought to keep.
I've been a creative on and off medication, and I've been both a writer and a non-writer on both. There's no one-size-fits-all fix for a problem like this, and truth be told, I have still struggled with this my entire life. How do you keep writing when everything in you is telling you to stop?
And I got a secret for you: I never learned how to shut the voice up. I learned how to do it anyway. Who gives a shit if it sucks? You don't have to show it to anyone. It never has to go anywhere, and if it does, you'll edit it. What you wrote might not make it to the final draft. What you write when everything is telling you not to isn't anything more than resistance, than fight, than stubbornness against something inside you that tells you "you can't do this". Just doing it is damn good. Let it be terrible for now. It isn't the final draft. Progress not perfection.
Since I've been medicated, consistently, writing hasn't gotten easier exactly. I still put it off because I'm overworked at my company and put in long hours between clients and internal work. Dealing with the voice in my head telling me I shouldn't write, though, has gotten easier. What used to stop me mid-sentence is as easy to ignore now, most of the time, as the voice that tells me I need to take out the trash.
What's the spark you're losing, if you seek help? A voice in the back of your head telling you how awful your work is? Sounds like a spark worth losing to me.