The thought of disappointing them puts you in actual physical pain, that's how bad you want them to be happy with your choices. They raised you. You respect them. They love you. They paid for school, maybe. You love them. And they have some very particular ideas about what you should do with your life. But you...don't want to do that.
Maybe you don't know exactly what you want to do instead, but it's clear it isn't what they are thinking. How do you get out of this situation? How do you respect them, love them, show them that you want them to be happy, while also doing something that will make YOU happy?
Why does this happen?
There are lots of reasons that parents have a clear idea of what they think is best for you. It might be because they feel they were limited by their own career prospects, or didn't have the opportunity to develop the security and success that can come with having an achievement-oriented career. Often parents who dictate for you what they think you should do are afraid themselves, afraid that you will suffer, struggle, afraid that their efforts in raising and educating you will be wasted if you do not go on to experience security and prosperity. Or, maybe it's because they missed out, and didn't pursue the career that they might have wanted to if given other options, and they may be living vicariously through you. Or, maybe it's because they themselves have been enormously successful, they know what worked for them, and they want you to have the same experience, or to demonstrate that you are part of the same in-group of successful people.
Largely, this is about security, their fear of your not having, their fear of losing it with you or through you.
Your parents and your relationship to them are important. You should not throw that out as you make your life and career decisions. But what I see a lot is that the fear of disappointment and the pain of potentially going against them gets in the way of having clarity and enthusiasm and joy around what fits you better. If you struggle to know what you want to do instead of what your parents want for you, it could be because the fear of their reaction to anything other than what they want is getting in the way. So have compassion for yourself and for them, and set that fear aside.
If your parents were to be in complete agreement and support for whatever career path you chose, what would you want to pursue? Imagine what it would feel like to have both the career that will make you so happy, and their approval, love, pride, and support. Pretend just for a moment that that is possible, and see if you can actually feel it.
Let them see you as successful, and happy
It's important that your parents see your successes and happiness through making your own choices. But demonstrating this requires a leap of faith on your part, and the bravery to act on your own desires and dreams, and to let yourself be successful at them before or without your parents' permission. So act on what you would like, take the first steps that you can to pursue your interests and ideas. Allow yourself the possibility of joy and success in that.
Everyone is different, and I can't promise that your parents will ever support you fully in walking your own career path. But to be true to their intentions for you - that you live a happy and successful life - may require that you step out along a different route than the one they have planned for you. Doing that takes courage on your part, but doing it out of love for yourself in addition to love for them and respect of their intentions for you, is the way forward.