@singinjunkie109,
Take a deep breath.
Anxiety is a perfectly normal reaction to stress, but it doesn't mean you have to let it overwhelm you.
First of all, rehearsals of 3 hours; five days a week doesn't sound overwhelming. It's not an easy schedule, but it's also not one that should overload you unless you feel you have to use that time for something else that is very important to you.
You indicate your main focus is your grades. Do you feel the rehearsals are preventing you from devoting the necessary time to your studies?
If so, examine it dispassionately.
I'm assuming that you are in High School, and if I am right than the end of your rehearsals should put you around 7:00pm.
How much time do you feel you need to put into your studies?
Assuming it's the three hours your spend at rehearsals, giving you an hour to get home and eat, you can still hit the books until 11:00pm.
Presumably the period of rehearsals is finite so jam packing your days and going to bed between 11:00pm and 12:00pm is temporary.
Can you manage this for another three or four weeks? (How long before the play?)
One thing I can promise you is that if you can stick it out and perform in the play, you will feel wonderfully exhilirated. If you don't (and that may very well be the best course for you) you will almost assuredly feel some level of regret, and that level may be quite painful.
Generally, we can all do more than we think we can, and almost all of us need to push ourselves past the point where we think we can do no more.
Chances are very good that you can manage this, but if you truly can't then you should move off from the play.
How do you know if you truly can't? Do you want to sleep all day and not get out of bed? Are you experiencing physical manifestations of extreme stress like skin break-outs, bowel problems, intense headaches?
Performing in a play is not worth wrecking your health.
The most valuable achievements you will ever have, though, are those which were difficult to achieve.
I think you can figure this out yourself, but it won't hurt to discuss it with someone you trust. Hopefully, your parents or siblings, but friends a favorite teacher, a pastor etc will do.
When you feel overwhelmed take deep breaths. Tense every muscle in your body for 10 to 15 seconds and then relax. You are experiencing the primal fight or flight reaction that has benefited humans for a very long time. Unfortunately, more and more we are being taught to fly rather than fight.
One last suggestion. Realize that eventually the time of the play will come. It's not something that is going to hang out there forever and ruin your life. There will come a time when the level of your stress is greatly reduced. The payoff for making it through this tough time will be great, and when it's all over you will wonder why you ever questioned your ability to do it.
Good luck, and let us know what you decide and how it turns out. I, for one, am betting you have what it takes to make it through.