From the moment of commitment nature conspires to help you.
— Bjork, "Cetacea"
Commit. Decide. Do it and don't doubt it.
You can't know anything for sure. Not if this scene should go ahead of that one, for instance. Or if the story should be first person or third person limited. It's not certain.
It will never be certain until you make a decision about it.
Don't wait until the moment you're certain before you commit to what you're writing. Commit now. Your work is to create something in the midst of uncertainty.
That's precisely why it's so difficult!
When you commit to something, your work will show you what needs to be done. It will help you. But it can't collaborate with you if you haven't decided to work with it yet.
What if you could trust yourself?
What if you decided that the first person point of view was a good one, and then just did it?
Decided that there are two sisters, not three?
Decided that it all takes place in Florida?
Decided that the chapter ends here? And so on.
Give yourself the benefit of the doubt. See what you can get done when you stop second guessing every creative decision you make.
Every book that's been written is the result of trust. There's no secret set of instructions that you missed - it's just a decision.
Commit to it.