What defensive purpose does a poor self-image serve? In other words, how does a poor self-image help people to feel in control of their well-being?

It’s defensive purpose is self-preservation – I am in control; I can gain power. It’s a reaction towards bad events/experiences that he/she is not in control over eg. sexual abuse, being lay-off from a job etc.
It fortified he/her against further hurt and pain, through ‘self-deception’ – choosing to believe in what’s not true.
God is not good enough, I’ve got to help myself. “God helps those who help themselves.’

A poor self image is a defensive mechanism whereby the person feels that by changing in some way for the worse he or she will no longer be as desirable etc as before and therefore will be safer in the world