I rarely see anyone multiple times and I've never forgotten payment so this is only how I think I would respond in your situation. I would have run right back over and made payment because it was earned and you have a p4p relationship. I would have been pleasant and apologetic about the mistake.
I would never contact her again and if contacted by her I would decline to see her again. Although I understand the p4p aspect and would do my best to not ever forget to pay donation after such a time I would expect to be treated with more respect for such an error from someone I have seen for 6 years. This is an easy thing for me to say though because I realize it is a pay for play situation, which is the only thing I am looking for from a provider so attachments are not their for me to begin with. You have to decide for yourself but it doesn't sound like you mean much more then $$ to her anyway even though it sounds like she mean's more to you.