Ahhh! My son is 6 1/2 and only after he turned 6 did he finally willingly (no, I take that back. He still isn't willing!) let me brush his teeth. I guess the big "step up" here is that I no longer have to be octopus armed and pry his mouth open, etc. I am REALLY looking forward to when he does it himself!!!!
Just because it is hard, dont back off. I did and regret it big time now. Because I didnt brush his teeth so consistently when he was an older toddler (and because I didn't give him fluoride) he had SEVEN cavities and a couple spacers in his teeth now as two were so bad they had to be pulled. :-(
It does and will (EVENTUALLLY!) get easier. She will get used to it. She has to. And you have to be consistent.
Suggestions - maybe a little timed piece of music or something where it only goes for 1-2 minutes and teach your daughter that you brush as long as it is going until it stops then ok, you're done.