4 Year Old Gags and Throws Up During Teeth Brushing

My 4 year old daughter has always had an aversion to brushing teeth and I understand a lot of other children also have a bit of a problem with it.For the last four months, I’ve been working with her on getting it to a point where she might cry a bit during teeth brushing but she no longer screams or throws up. I diligently do this every morning and night even though it’s unpleasant for the both of us.Her mother (divorced) has historically been very lenient with this act although she insists she brushes teeth.As background, the mother has been very...undoing of behavior conditioning and when we were together, would undo any kind of corrections I would give to my daughters.I see my daughters from 40%-50% of the month consistently throughout the month so that there’s never longer than 4 days before I see them.Over the last two weeks, they have been with their mother, however, and it seems like all of the progress made with teeth brushing has been COMPLETELY reversed.Last night and this morning, she yelled, fought me, refused, and ultimately gagged and threw up. So we had to do the whole thing again including breakfast and teeth brushing.It’s extremely frustrating to be back to square one. It’s easy to see it’s a mental thing because as soon as we say it’s teeth brushing time, she gags and we are in the living room! She used to look in the mirror while it was happening and became obsessed with seeing herself crying. I noticed this and started brushing away from a mirror and it was a success! Suddenly it became very easy - until today.What can I do to get back to where we are? Is it punishment or mental? I don’t believe that her mother actually brushed her teeth properly every morning and evening. One, because she’s terrible at discipline, and two because she wouldn’t have reverted back to square 1 if it had been consistent.This is literally the worst part of my (and her) day and I know it can be better. HELP! Anyone else have this problem?*****EDIT: Update, had a conversation with her mother who said that she had indeed been doing a good job for her and ALSO brushing her own teeth sometimes too. They went on a trip recently and out of the blue, my daughter decided the first night of the trip that she was going back to square 1 because there wasn’t a step stool to see herself in the mirror. Additionally, her mother said she has to hold her arms down and press her head against her side in order to brush them. A strange side note is she’s very compliant when she’s being held tightly or down and we are now worried it might be a strange obsession with being restrained. Anyone able to relate? via /r/Parenting http://bit.ly/2F3RtZO