I have a very similar problem and I agree with Jill that, in my case at least, the cause is a lack of self-esteem.
I've become quite good at graciously accepting compliments (well I get so many, you know...
) but its very rare that I accept them within myself.
In my case, I usually feel pretty certain the person is just saying those things because they think I need to hear them, feel sorry for me, etc etc. The very last thing I would consider is that they might actually mean it or that they might be right.
A very interesting awareness to develop is the gap between how you see yourself vs. how the world sees you. I once had this experience, through the process of group feedback, in a self-development course I went on. It turned out I was an ok human being. The relief!