A lot of people swear by MTG, but I have no experience with it.
However, I would suggest that you take that halter off and leave it off. If she must have a halter during turnout due to barn rules, buy a larger leather one and put padding on it. This halter could be too small for her also, just as it sits. If you don't rectify it, she's going to get sore and then not allow you to halter her and react poorly to being ground worked, then also riding because the bridle with bother her as well.
If she is hard to catch, consider a leather neck band instead of a halter.