"I can't write"
"I need to have this chapter perfect before I can start the next"
Write that book, story, article...
Finish that chapter! They're called rough drafts for a reason. Get your idea down on paper. You'll be amazed how great it makes you feel.
Join a critique group - either online or in person
Learn your craft ~ go to conferences, take an online class, find a local writer's group
If you take care of the "Should Haves" than you will have less "Could Haves" at the end of the day.
Keep writing and rewriting,