Why isn’t voluntary euthanasia legally available for anyone who is an adult?

I’m soon turning 18 and I don’t want to have a life to have to be responsible for. I’d rather simply not exist and would like a way to leave this world that’s painless and won’t risk me ending up disabled or something instead. Since this is my life, I don’t see why I shouldn’t be able to have this right if I’m able to find a doctor willing to assist me in the treatment. I don’t see any reason why people shouldn’t have every right to die on their own terms whenever they may wish to do so (with certain exceptions, for example if you have a child depending on you). Can someone explain this to me?