I think of this a "performance anxiety." Like making yourself sit down and actully type the first word of a story or rip the arms off a fur coat to make a mink teddy bear.
So, I start in the middle! I don't write the first word--I write a scene in the middle of the story that I see as plain as day, and i rip an old mink coat in half and get it over with. Then it is just a tool of a process that is half over-see--not precious fabric to "save"?
Maybe you could play like that with your "planned quilt.'" as if it didn't matter where you start, just get going.
So, ME? I would take the design I like to tackle and and make a sample of it in blue, teal or green (my "oops" colors) work out the angles and hassles on it, and then go through my stash for colors/designs I really love. Settle on 3 or 4 that you have to have in that particular quilt or NOT to use to save for another quilt, and then rip the chosen fabric in half!
Yes, you may have to piece your back - who cares? You will be able to get there.
Then you are free to create.
Works for me.