Once upon a time I studied to become a professional musician. I learned some core tenets of "learning," or maybe we can say "improvement," from that time. I would recommend everyone spend time in formalized study of some kind of instrument.
The biggest, most important thing that I learned was the value, no, not even the importance, but the requirement, of daily practice. I started out as a kid not practicing at all or very sporadically. As I grew more serious and passionate, I began to regularly practice 30 minutes a day, most days a week. Sometimes I had bad weeks, of course, and only practiced 3 or 4 days. The thing was, every day I practiced could be heard in the way I sounded at my lesson at the end of the week. I started practicing nearly every day for 30 minutes. Then every day for 45, then an hour, then 2. Yup, by the time I went off to college it was de rigeur for me to practice every day for one and a half to two hours. And it didn't even hurt. And you could hear it.
What I learned, over the course of this 'transformation,' was that how I felt when I started my practice session each day didn't matter. I could feel like crap. I could want to practice, I could hate practicing with my entire soul. I could feel great. I could really want to practice one thing, I could really want to do nothing but scales, whatever. No matter how I felt, whether I put 100% of my energy into that practice or not, simply doing it had an impact, and a positive one, on how I sounded at my lesson at the end of the week. (Which rolls into "how I sounded at my concert, how I sounded the next year, etc, etc.")
I had terrible practice sessions where everything went wrong. I had great ones. I had ones that I started with a black mood about everything but things still went right. I had things I started positively and I still tripped in the same place I'd tripped last week. None of this mattered.
What mattered was putting time in.
It was never about "being inspired." It was never about having "musician's block." Those don't exist and don't impact, aren't allowed to impact, a musician's day-to-day. Simply put, I had to put the time in, and nothing else mattered. But god damn, no matter what, putting the time in got results. Often I felt better at the end of a session than I did going in. Often it was more rewarding, more interesting, less boring, etc, whatever, than I expected. Often those 30 minutes flew by way faster than I thought they would, even though I'd done them yesterday and the day before and you'd think I'd have realized it by then.
By the end I no longer fought against practicing, mentally, because I realized it didn't matter so long as I put the time in. I realized it wasn't that bad, in fact it was pretty great and helped me. I started to find it meditative and relaxing in ways. I gave up, most days, on the bad attitude because after years I realized the bad attitude was just a hurdle trying to stop me from something that would not only make me feel good when I went to bed that night but had a definite, measurable, positive impact on something at which I wanted to succeed.
I have learned nothing so valuable as the lesson of practice. Of simply putting time in. You can throw shit down on paper and yeah, it's shit. But you did it.
I try to never feel bad about putting shit down. I need to write a lot of shit out before I get to the good stuff, sometimes. I'm still writing. I feel that even the shittiest writer can improve in writing by doing it every day, simply because they become more familiar and comfortable with using language, the mechanics of language as well as writing, with the thought process of writing.
So far I have not come across an arena yet where this philosophy: the value of daily practice - is not only applicable but immensely valuable.
I think you should try to silence the voice in your head that is saying, when you write, "This is shit. This is shit. There's no point. Just stop now. You're an embarrassment. You're a failure." Or better yet. Don't silence the voice. Say, "Maybe it is. But so what?"
You've said it to me yourself and quite recently: how can you hope to be a success if at first you do not fail, at least a little? Let the shit spew onto the page. Sometimes, it takes you for crazy interesting rides. Just do it. And do it tomorrow. And do it the next day. And do it, and do it, and do it.
Buy a timer, set it for fifteen minutes, put it where you can't see, and write until it goes off. I like the 15 minute comment. I think it can be applied to this. 15 minutes is small and not scary. Try it.