I had the same thing to a certain degree still do. My parents were always complaining about me not being good enough and how much more perfect my older sister was. I don't think it can ever really go away. There were a few things I did to lessen the strain it put on me physically and mentally.
1) Space. If you can move away from them, even just for a little while with little to no communication, do it. If they are always breathing down your neck you can't cope.
2) Rant, Rave, Clear the system. Get all the anger, guilt, insecurities out. It helps to do this once in a while rather than just the one time. Especially if you can't talk to anyone about this, talking into an empty house or room relieves the mental build up.
3) Job and Hobbies. Find as many things as possible that you can put your all into. For me, I started the writing career I had wanted for years. I also do a lot of reading, play some video games, sketch, and do domestic responsibilities.
4) Look at yourself in the mirror and describe the exact person you want to be. Don't put in other people's expectations of you, just you and your desires.
5) Follow through. If you want to change your appearance, do it. If you want to pursue a creative career, give it a try. If you want to ask that hot guy/girl you've been eyeing since high school, go for it. As long as you are stable and financially secure what you do should not matter to anyone but you.
Those are the steps that I went through and am still going through. It's a tough battle but if you really want to be your own person and stop feeling guilty over other people's disappointments you'll get through it with flying colors.
I hope this helps. Good luck and i know you'll do just fine in the end.