This is not a question which can have a satisfactory answer because we're not in your head and because different people deal with writing in different ways. My best guess is that you need more practice or need better coping techniques.

I used to have similar issues, in large because the idea wasn't as well thought through as it seemed in my head. I resolved this by use of a few techniques and through practice.

The first technique was to talk through my paper with someone. When you're explaining it to someone else you'll think it through in a different way because there is a lot of mental shorthand you use which will need to be explained to them and often you'll find inconsistencies through such an explanation that you might have glossed over while only thinking about it. This is also why I believe that teaching a subject is, at least for me, one of the best ways to really learn it.

The next technique is actually write up your outline. Each paragraph gets a line in the outline devoted to it. When you get to writing your first draft each paragraph starts with its own thesis. Each paragraph ends with something that ties the thought back to your main thesis or into the next paragraph. These can be a bit sloppy on this draft.

Finally, your first outline and your first draft are really only there to make your second outline. Stuff won't quite fit together in the first draft, but you've got most of the points down that you want to cover and you can rearrange them so they fit together into a cohesive thesis. Then you again write out your outline, this time with a word for word thesis which follows that outline. Then and only then can you write the real paper. You'll be able to reuse some whole paragraphs sometimes, but often at least the first and last sentence will need to be reworked to fit together. Other times whole paragraphs will be struck, added, or reworked entirely.

Even then these are just the techniques which I eventually got to work for me, different people need different strategies. After i found my strategies I needed more practice and the same is likely true of you as well.