ec,
I met a LOT of discouragement when I began to tell people I was considering applying MD/PhD. Probably more negative feedback than positive feedback. However, I took it all with a grain of salt, and found more worth in the few positive pieces of advice I received.
The bottom line is, YOU know what you want and YOU know what you are capable of much more so than any one professor you have. Although it is always good to collect feedback and differing opinions, it's not always good to give them too much weight. If MSTP is what you want to do, please do not give up and be discouraged. MSTP applicants are often very self selecting because know (or have a good idea) whether or not they can cut it and whether or not it is what they really want. If it is your goal, stick with it, and look at this as an opportunity to show pessimists that they were wrong about you and that you will succeed! You'll do great. Good luck!