You are definitely not alone. It's a really big boat, and we're in it with you, life jackets on. Paul Lamb is correct about daydreaming; It's so important to open your thoughts and just let whatever comes take hold then use those thoughts.
Maybe some objectivity would help too? Have you heard of the left brain/right brain thing? Betty Edwards in her book Drawing on the Right Side of the Brain explains it so well. Her book is for people who want to learn to draw and "see" how artists do, but her theory applies to us writers too, I've found. We're right brained people. When writing, we live in the right, creative side of our brains that believes anything is possible, that black text on a white page can make perfect strangers feel like they're our characters. The left side of the brain, the logical, "that's not real because your character isn't a real person," fights us every step of the way. It plants doubt gremlins, and once their nasty little teeth grab on, we're well on the way to thinking our writing and ideas totally suck. Maybe when that first tension at an idea hits, tell yourself it's just doubt gremlins, the left side of your brain objecting. Daydreaming then writing is an excellent way to shut Ol' Lefty up. Also, like Betty Edwards suggested free drawing (emptying your mind, letting the pencil or brush move across the paper without thought or direction), free writing helps. Maybe concentrate on something that has nothing to do with your WIP: a shrub in bloom on the lawn, your dog's face...whatever. Open your mind, think of all the descriptive phrases and words you can for your subject then write a description, stopping only when all the words are out of you. Seems like, for me and some of the authors I edited for, anyway, just the act of writing words (black text on a white page) satisfies Lefty because we're actually doing something logical and left-brainy. Leave that bit of writing for a few hours, maybe even a whole day, then read it. Don't nit pick re: punctuation, spelling or grammar, 'cause those are left brained things. I really think you'll be thrilled at how worthy your writing actually is. Every word and phrase that strikes you as good, stop, congratulate yourself and say your version of, "I wrote this. And it's freakin' good." It gives you confidence, and confidence and recognizing that it's just Ol' Lefty butting in, messing up your mojo sends those doubt gremlins into their stinky black hole. I do the free writing thingy before I sit down to my WIP, then just work on my WIP. Lefty seems to stay asleep for me that way. For you too, I hope.