I think Eripon's problem is more of an identity problem which can relate to the lack of confidence. Obviously, she knows she has fans, but she want to improve and has her own stand in the group, where everyone can say, "that's Eripon of Morning Musume," like, "that's Riho of Morning Musume." Right now, she probably feel like she is a side member that can just not be there. Kanon and Harunan probably feel the same sometimes, but the difference between them and Eripon is confidence. I think deep down Eripon is still very sensitive like when she just joined, but because of pressure from her environment, senpai, kohai, management and fans, she is forced to have a personality and be confident, which does not mean she is actually being confident. Psychologically speaking, she use a fake personality as a self-defense mechanism. In general, you slowly gain self-confidence by doing that, but you cannot avoid being criticized, and how much confidence you will gain depend on how well you response to those criticism. I see Kanon as a self-sufficient girl while Harunan is very mature, so they can handle criticism well. On the other hands, Eripon is more sensitive, so the effect of psychological self-defense is reverse. If you check her blogs or listen her radio appearance, you have know for awhile now that she has given up on her fake personality thing and is looking for her identity in the group.
I think what Jessi said is right. The best cure for her is support from the fans. We just have cheer the real Eripon her enough to the point where she has enough confidence in her self. I think Eripon is a very cool girl, friendly and (now) courage, so if you like those side of her you should cheer for her. It is also good that she uses Sayu as her inspiration, and thinking about being a good senpai rather than just being in he center. She might have just do that because Sayu's popularity surges tremendously after she proves being a good senpai. But that doesn't matter. The key is the more people who love you, the more confidence you will have.
Sorry for a long, somewhat academic post.