Do you really want to grow up? Then quit doing the same things that have got you in trouble all your life. You quit smoking, drinking, and have so much going for you, but you chose to worry about people that will destroy you. If you don't say No to the parasites that want to use your car, borrow your money, make fun of you and drag you into their meaningless existence you will be a screwed up mess. I know you don't want to get picked on, and I have been there myself, but honestly stand up to evil.
You need to respect yourself, and I can quit worrying about you being too nice and getting bullied. A friend that cares about you, I remain.