Hellion, it might be easier to just go to your regular doctor for a checkup and tell him what you're up against and see what he recommends. Just decide you want some base level tests to make sure you're physically okay and explain to the doctor, that because of your age, situation, and no one seeming to be hearing you, you don't know how to proceed in your particular situation, what would he suggest you do. He might have a referral for you and that would make it easier, you could just tell everyone you were following-up on what the doctor said, that might have enough "authority" so they'd be able to hear better.

I don't think your parents and other close relatives want you to be ill, that might mean to them that they've failed you. So, they're trying to reassure you (and themselves) instead of looking at the overall picture (your suicide attempt at 15, your dropping out of college, and your complaints now). I would get someone outside the family involved, someone they'd respect to help you get some treatment. Are you in contact with whoever you saw when you were 15 or could you go "there"?