OK this is a topic that I have had cause to come across. In one situation you have the person who is beyond help and must only be helped by a professional. A fellow Dom I know once tried to Top a girl who was completely into him. This sounds like the perfect situation bar the "Fatal Attraction" it almost lead to. she was totaly beyond help and while he thought he could help he was completely wrong. If it looks bad - get out quick!!!
Then you have the other situation which I have come across. When the sub is aways putting him/her self down.... I find that this type of low self esteem can be overcome by being Dominated!
Any sub who gives anything of themself to a Dom should understand that the Dom has the best interest of the sub in mind. (If you find your Dom does not then maybe your being abused - get out). If you went out and bought a car the first thing you do and not look on how to crash it (at least I hope it isn't - except for the film "Crash"). What I mean is a sub is your responsibility and property, to be looked after. If the sub has low self-esteem then the whole D/s relationship should be able to fix this with the constant knowledge to the sub that it is doing well. Pride is a powerful weapon when it come to low self-esteem.