I'll second the tip about starting with a shorter, more manageable project! I've had the plot for this huge epic fantasy story in my head for like 10 years now, have draw the characters hundreds of times, revised the plot heavily now and then, and drawn the first like 10 pages at least 6 different times. It never took off.
I had just been working on it again this January when i came to the conclusion (after reading a lot of threads here) that I should start with a shorter one-shot comic to get my feet wet! I now have a short like 66 page comic 100% story boarded out and ready to draw, that I'm hoping to finish either at the end of this year, or early next. In fact I just penciled the first page last night!
There are 2 really cool things about it:
1.) Even though it's in doodle story board form, I have a whole complete story that I wrote in a PDF that I can scroll through. It's a really cool feeling!
And 2.) I know exactly where I'm going which i think will make the actual work easier, mentally. I managed to pencil out the first page in like under an hour last night thanks to the story board where I already worked out a lot of the challenging lay out/angles/etc.
So yeah, I'm excited to get started, but i don't think i would have been able to very well with the first story. I'd like to revisit it someday, but for now going to do some short -medium comics!