Don't feel bad! Even professionals do that!
The writer gives the script to the drawer, and sometimes he changes things to make it look better, other times the writer change the dialogues to be better understood at final time, and then we have the marvel system, where you only gives a synopsis to the cartoonists, they do everything, with empty balloons, and the writer fill them with anything at last minute xD
In my case, I did the whole story with arcs, and every chapter have a synopsis of what should happen, and just at final minute I write the chapter page by page. Then draw the thing, and fix anything that should be fixed. That way I have an script flexible enough if I add something (I have done that), but stable enough to know where is the north of the story.
Good luck with your scripts!