It is common for people to be told, "Your problem is that you have a low self-image." Such statements usually lower a person's emotional state and make them feel worse about themselves.
It would be much wiser and kinder, and more beneficial, for people to say, "You have infinite value and worth. You already know you have strengths and inner resources. But you have even more strengths and resources that you are not yet fully aware of, and they will enhance your life as you become more aware of them. There are many more strengths and inner resources that you can gain and build up from now on. As you keep building your self-image, it will have a positive impact on all your future thoughts, words, and actions. You will be calmer, happier, and more self-confident."
(From Rabbi Zelig Pliskin's book: "Building Your Self-image and the Self-image of Others" (Artscroll) Introduction)