My DD told me she wanted to quit this spring. I really was very upset because, let's face it, we have a lot invested in the sport too, and a lot of our social life ends up revolving around the gym. However, my daughter's happiness and well-being is my #1 priority. I tried VERY hard not to let her see how upset I was. In the end, we met with the coaches and she decided to stick it out for another year, but I was prepared to support her in whatever decision she made (and truth be told, by the time she made the decision, the little calculator in my head was adding up all the money we would save if she quit).
A few things based on my experience...
My daughter sent me a very well thought-out email outlining the reasons why she wanted to quit. That was a great idea! However, then be prepared to TALK to your parents about it. They will want to know how long you've been feeling this way, what you plan to do instead, etc. This is because they want to know if you are making a hasty decision that you are likely to regret. Gymnastics is HARD to get back into once you quit. Getting my daughter to talk about quitting was like pulling teeth. I understand that it is a difficult topic of conversation but parents really want to know what the thought process is behind your decision and that it's not something that occurred to you for the first time yesterday.
Also, you will likely need to talk to your coaches. They will probably try to talk you out of quitting, so if it's something you feel strongly about, go in there ready to stick to your guns.
Ultimately, I will be sad when my DD quits, but if she were to come to me when she's an adult and say, "I really only did gymnastics because you wanted me to. I was miserable all through high school." I would be devastated. Your parents love you enough to support you through this crazy (and crazy-expensive) sport. They love you enough to support you in quitting it too.