Nearly all writers procrastinate. The ones who don't are on some kind of deadline! Either publisher-induced or from some outside requirement. How do I know this? From talking over the years to many, many writers, and observing myself. You're not alone.
Procrastination is a manifestation of different things but the one biggie is fear. Fear that what you write will suck. That what you write will cause some kind of upheaval. That what you write will cause you to be REJECTED. I do think that 95% of the time, there is a very direct line between procrastination about your writing and your fear of it being rejected.
It doesn't matter by who. It is likely to be a publisher, but can also be any or all members of your family, your spouse, your second cousin twice removed that you used as a character because she is just so weird. The thing is - it's in your head. And the only person who can get it out there, lay it on the table and dissect its cause, is you. A lot of writers either don't realise this, or don't want to do it.
But what happens when you stop procrastinating? When you actually shove aside every excuse, reason, fear or "block" and write?
You write. And you often write good stuff. You end your writing day feeling terrific. Feeling like a million dollars. Feeling like "why did I spend half my day avoiding that when it was so GOOD?".
Next time you write, and you have that great feeling, this is what you do. You take a few minutes to describe that feeling to the best of your ability. You use every descriptive word, you explore the feeling, you can even draw pictures of it. Then you put it up above your computer or your writing space so the very next day, there it is. You read it. You remember what it was like to write, how good you felt, how the words flowed out despite your struggling.
You read it several times if you need to. And then you write again. Use that feeling. Over and over, use it to remind yourself that yes, writing is hard, but when you do write, the writing itself is the best reward ever. Make it part of who you are as a writer. Celebrate the writing.