Wow Leaf, you are truly an amazing boy I must say! Given that you have went through so much emotional pain (I'm sure your parents have downloaded theirs to you too) I'm impressed with such maturity you displayed in handling those events and never using that as a legitimate excuse to go astray.
Taking my hats off to you Leaf!
I agree with what Erin said too. Do not give your parents permission to bring you down. At the same time, maybe you can consider leaving some books on PD or positive thinking lying around, on the couch or coffee table or something. Who knows one fine day your parents may just pick it up and start reading it.
It takes time for your parents to realise what they are doing, for what I see now there're lots of emotional blackmail. If you really want to help them get out of that rut, firstly you have to stick on with your growth and personal development that have started to bring you those good things in life - relationships, friendship, healthy self-esteeme etc.
When your parents see such glow in you for an extended time and they knew that will last, they may just be curious to know what causes such change. So the books may just come in handy!
I tested this before (though on a different situation) and it worked! My mom actually read that book I left there. What she didn't realise was that was left there deliberately, rather than carelessly as what she assumed.
Lastly, all the best to you Leaf. You certainly deserve more good things in life, esp love and happiness.