Very common. It’s like writers block. You have to train your mind through it. In ten years you will have no problems what so ever. That is if you force yourself through those ten years in between.

That’s how long it takes to master something you are naturally resistant to.

Start with a journal, preferably hand written. Just sit down every morning, first thing, and any other time of day you can make it a ritual, and start writing.

Just write and write and write at first, and gradually structure for whatever you want to actually get done.

It will make you a better thinker, all around. Overcoming personal barriers always does, whether it is “getting started”, socializing, public speaking, whatever.

Just make getting to it your ritual and over time it becomes natural.

Maybe you're a verbal processor? When you're alone, do you talk aloud? If you don't, consider trying it.