I'll tell you what a friend and I did for another friend whose maiden mare foaled and wouldn't let the foal nurse. We haltered the mare, tied her up, and one of us held up one of her legs while pinning her to the wall while the other guided the foal to the right area. This mare was NOT wanting her bag touched and she was really awful to deal with, it was quite a fight. We got the foal to finally nurse a few times, but it didn't survive.
I don't know if it didn't get colostrum early enough, or if the mare continued to fight against letting it nurse.
I always desensitize my mares to having their bags touched and handled, so that when they do foal, they are fine for the foal and for me to milk them if needed. It can take months for some to get used to it (and others are ok from the first) but it's really important, IMO.
Good luck with your filly, and please do have her checked for IgG levels by a vet.