Okay...after reading your posts, and all the responses, I'm going to see if I can't offer a little advice.
First of all, just because you make a few B's or C's does NOT mean you can't hack it as a doctor. I personally know many doctors who talked with me about how they struggled with a lot of their premed work, and there are numerous other posters on these forums with success stories after having some rather poor premed grades. Furthermore, not many people look forward to taking the MCAT, other than just to get it out of the way and be done with it. Now, I don't know your intellectual capacity, nor do I know whether you actually can cut it as a doctor. Only you can know that. But based on the information you gave, it definitely can't be said that you can NOT be a doctor.
Now, do you owe your parents for all the sacrifices they have made for you? I would say, yes, you do owe them. To anyone who says that what your parents did was stupid, that just shows their ignorance, and don't listen to that. Based on what you have told us, your parents did everything they could to place you in an environment that would give you the most options to be successful. Did it mean that they had to work extra shifts, wear themselves out, and give up other things they may have wanted? Most definitely. But that is called self-sacrifice, and that is what being a parent is about. However, does all this mean that you should be a doctor/lawyer/engineer just for them? No. More than anything, based on the sacrifices they have made, your parents just want you to be happy, and have opportunities for things in life that they didn't have. So, to truly repay them, find something that you enjoy (and this is key - look around while you are in college and are able to do so), and COMMIT to it. I believe once you commit to it, the "passion" will follow.
I can relate to your situation, as I am currently dealing with the same situation. My parents have sacrificed greatly on my behalf, and for this I feel a responsibility to them. There is nothing wrong with that. However, they have not pressured me in to medicine, but I originally presented the idea to them. Since then, they have backed me 100%. However, I am doubting whether medicine is really for me, and am most likely going to pursue another career. While they definitely had questions when I told them, and they were trying to make sure I thought everything through, they are starting to come around.
So, to sum up the book I have written, you do owe your parents something. But don't choose something that will make you miserable. Find something you enjoy, and commit to it fully. When you do this, and communicate it clearly to your parents, they will deal with any disappointments they may have and will be happy for you. That's how you pay them back.