The only advice I have is to develop a routine. I brush every morning and evening about the same time. I also feed at the same time. I would start with developing a routine for the clothes as well. Maybe start with leaving on for an hour? Then increase the time? The routine works for mine. Hopefully it will for you too.
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As far as brushing underneath, have you tried holding her up and then brushing? Or having someone else hold her? Wish I could give you more suggestions. Good luck!