Your parents should have cut you off a long time ago. Your story, from all that I've read, reflects in startling detail a good friend of my own. The specifics do not matter, but this guy just continues to bang his head upon the fault line, and is able to do so because of someone paying his bills. Whatever your situation is, and the reasons why they do this for you are beside the point, and have changed drastically from the initial conditions in which they were set. If you really want to get clean - successfully clean - you should tell your parents the truth. This seems like a direct course of disconnect from the financial support which enables you to continue your many addictions. It would be truly sad, and harmful to your health, if they did not do so. Your fear of your parents does not lay in their perception of you, but rather in your wallet - Your shit is backwards as hell, man.
P.S
Look into checking yourself into a hospital. I have overcome addiction, but through different methods, so I am not at all certain if this is the same as rehab or just a public health service that you should take advantage of; I'm sure the community knows. I don't wish 'good luck', and I doubt that you'll take this advice; although you really should.