I do think it's not okay for parents to try and force their children to fit a particular mold. A friend of mine was in the same boat--her parents wouldn't pay for college (university in the US) unless she did what's called pre-med (preparation for postgraduate medical school)--and guess what? She never went to college. Idiot parents. The world does not run on doctors alone. We need all the professions.
And so I want to go one step further than what Totoro is suggesting. There may indeed be, as Totoro suggests, ways to get your parents to understand. But there may not be. In short, there are three possibilities here:
1 - They may not understand now, but start understanding if you explain it to them enough and spend enough time talking with them about it;
2 - They may not understand now or after talking about it, but, say, 5-10 years from now when you've made yourself a good life they will understand (in other words perhaps talk won't get through to them but actions/results will); or
3 - They may never understand.
Personally my vote is on (2), because it sounds like you've talked to them quite a bit already without their experiencing any change of heart. Their manipulativeness (threatening to withdraw financial support, already withdrawing moral support) also doesn't bode well for a mere conversational clearing of the air. And while I hope it's not the case, there remains the possibility that they'll never understand.
I'm not trying to depress you here, Rory, and I hope it's not depressing. What I'm saying is that it's quite possible that you at a bare minimum have several years to go before even one, much less both, of your parents have a change of heart. And I'm telling you that because I want you to know that even though you may not get what you want from your parents or may not get it for a long time, still, however remotely, however far away we are, all of us here support what you're trying to do (namely, make a happy life for yourself--our interest in money is solely because, as Denzel Washington once said, "Money can't buy happiness, but it makes a hell of a good down payment"--or as we see it, if you have enough money and use it wisely, then you can spend your limited time on earth doing what makes you happy and what fills you with a sense of meaning, instead of punching a clock just to pay your rent).
So... I wonder if it would be worthwhile for you to imagine what you would do if, say, for the next 5 years your parents didn't change their attitude at all. First: notice that you do not die! Haha! Their lack of approval doesn't kill you! Second: notice that you could invest a lot of time and emotional energy in trying to change their attitude, or you could--this is possible--let go of the desire for their approval, accept (truly accept) that they are flawed, disengage (in other words if they start harassing you, put down the phone or leave the house), and thus have all that time and energy left over to use in building yourself the sort of life you want. Third: notice that everything in life that matters--friends, the quest for (or work in) a satisfying career, your growing self-reliance, good movies, good books, interesting places to travel, falling in love, etc.--continues to develop; giving up on seeking support from your parents doesn't stop it! And fourth: just keep on noticing...
And through it all, you will find people, here and on other forums and in real life, who will support you. Of course you'll find naysayers too, but there are MILLIONS OF PEOPLE who understand that our time on earth is short and it's incredibly foolish to yoke ourselves to a career that makes us miserable.* And unfortunately there are also lots of people who understand what it's like to have unsupportive, manipulative, toxic parents--parents perhaps driven by their own fears and their own wasted ambitions (did either of your parents want to become a doctor and not have that chance, or choose an easier path, or...?), and who let those fears dictate how they treat their children. It's terrible, and many people have temporarily or (rarely) even permanently strictly limited or even eliminated contact with such parents, because it just comes at too high a cost. (In a word, your sanity is more important than your maintaining frequent contacts/visits with your parents.)
Excuse me for sounding incredibly American, but we are rooting for you!
* I remember reading years ago about a dentist in his early 40s who had an epiphany while he was at the mirror shaving one morning: looking at himself, feeling miserable at the prospect of another day drilling away at people's teeth, he had the thought, "What right did I have, at the age of 17, to force myself to be a dentist for the rest of my life?! I DON'T HAVE TO KEEP DOING THIS!"--and he quit. Boom! I love stories like that, but I love even more stories like yours, where people don't wait until their early 40s to wake up...