Karin, looks like you're well aware of the "parent trap". If you choose a career to please someone else, you almost certainly will be unhappy with that choice. When you accept someone else's vision as your own, you still have made a choice. I went down that path a time or two, so I am very familiar with how that feels. You will be unhappy with the career because either it's the wrong fit, or you'll be unhappy with how you came to it, i.e. by someone else's will over your own.
Parents are primarily driven by fear. Fear that you will not be safe - financially or physically and so they make decisions or give advice to protect their children. They make up stories that if, for example, you're a doctor, or a lawyer, that you'll be safe. We all know this is not true for a number of reasons, primarily, if you do something for the wrong reasons, you won't be good at it. But, parents can't help but give this advice or pressure. There are also cultural norms and family expectations that they've inherited as well that come into play. Those are all based on that same fear paradigm.
Here's an example of how you can bring your parents around to at least being open to other things:
Parent: "You should be a doctor".
You" "oh yea, why is that?"
Parent: "Well, you're good with people".
You: "So, if I did any other work with people, would that be o.k?"
Parent: "well, no, because a doctor is secure and makes a lot of money".
You: "O.k. so, if I made enough money so I felt secure, would you be o.k. with that? "
Parent: "well, being a doctor is prestigious".
You: "but if I don't care about prestige, would me doing something else be o.k. with you? "
The key here is not to be defensive or angry, to try and understand why they feel the way they do, and to make sure you use "I statements" to reinforce what's important to you. "enough money so I feel secure".
Because that's ultimately what it's about. You making decisions you are comfortable with because you have to live with them. Good luck!