My mom refuses to let me cut my hair?

My hair is really long, about my mid-back, wavy, and frankly very heavy and hot. Its not healthy, its very dry and has split ends, and gets very frizzy in the back. It looks like a rats nest is forming at the bottom. Its summer now and i asked if i could get it cut, she didn't even let me say how short and said no. I explained that it is MY hair, i'm fifteen, and therefore should be able to at least propose to her ideas i have. She still said no because she loves it too much. Yes. SHE loves it too much, and said if i want it short, i should just put it back in a ponytail. I've been doing that for more than a year now... She simply doesn't understand i am nearly suffering with it and would like something different because i have had the same style for years, and have grown to hate it. I told my sister (who also has very long hair and is also a parent), and she said since its summer it would be good to get a new style. Its not like i'm going to get a mohawk or a pixie cut. She said i could do whatever i wanted with it just NOT cutting it! She is missing my point completely and i think it's very unfair. Please help, how can i convince her (and please, i don't want to go out myself and get it cut & dyed without her knowing) to let me get it cut. Thanks in advance