If you are in the U.S., even though you are not enrolled in the public school system, you can still call your local High School and ask to speak to a counselor and explain your situation. Tell them that you are being home schooled, that your family is very religious and you can only associate with those of the same faith but there aren't any kids your age you can trust. Explain that you feel trapped and would like to know what advice or help they may be able to offer you.
Before you do that, and if you think it wouldn't make things worse between you and your parents, you could write a polite respectful letter to them and explain you feelings, just as you've done here. Putting your thoughts in a letter and letting them read it may diffuse the emotion that sometimes gets in the way when you talk to them face to face. I wouldn't say anything about your feelings about the existence of God or about being a JW because this is a scary thing for religious parents to hear. You might mention that you have prayed to God but you don't think he cares about you because he doesn't seem to help in the one area you need help with. Tell them that the only thing you could think to do is to write them this letter and maybe that's what he wants you to do. Tell them that you appreciate everything they have done for you (give examples) but your are feeling trapped and it's effecting your mental well being. Let them know that you don't want to go wild or do anything wrong but you need to be around others your age so that you develop normal social skills and outside interests. Remind them that the Watchtower advises JW's not to isolate their children from the world, instead they should insulate them and that you are feeling isolated. Tell them that you are concerned that you won't know how to act when one day do have to go out into the world. Just remember to be respectful, mature, don't cast blame, rather suggest solutions. Tell them that you want to go to the public school even part time if only to take an art class or some other subject you are intrested in. If you think it wouldn't be too weird, you might even suggest that you'd like them to answer you in a letter as well, just to keep emotions from getting the better of you all if that has been a problem in the past.
Remember, plenty of non JW 15 year olds who attend school and have friends and go places, still feel the way you do. They also have to deal with snarky gossiping friends and parents who they think are overly restrictive. It's part of being your age to be somewhat discontent but it sounds like you do have some legitimate reasons to feel isolated.