I'd just keep doing it all over and over again. Keep a halter on underneath and then put one on top, take it off and do it everywhere. In the stable, in the arena, in the paddock. And if you're going to be doing stuff like that, hold her, so if she pulls back you can follow her. Work out a routine, so your horse knows what you do. Like if you like to loop a rope around her neck to secure her before catching her, then do it in the stable even when you don't need to, or in the yard, in the paddock etc.
And don't stress too much! It will come with time!