How can I make my parents realize that I'm an adult capable of making my own decisions?

Oh sweetie, I know this is difficult. I am with you, you're an adult, sounds like you've got a good head on your shoulders and are a very strong woman but you don't want to disappoint your parents. This makes it hard. You are right though. You are independent and have every right to make your own decisions. They have a right to give their input, but not to control you. They should respect your decisions, no matter how crazy they think they are, or how stupid, or how much they approve, they need to respect you and let you live your life. I hate to say it, but they may have to be mad about it for a while, then later on, see what a smart girl you are (even if you make mistakes and screw up your life royally!) and be there for you anyway. THAT is their job. Not to control you or give approval for everything you do. Just trust in yourself and it will all work out. There may be trouble for a time, but it will get better. It sounds like they love you very much and just worry about you. They feel they need to protect you, but don't realize that you need to stand on your own feet. Good luck to you!