To say that someone is ‘insecure’ is to say that their assumptions about themselves are tentative and reliant upon real time feed back from others and self-examination in the context of their existence in society.
In contrast, someone who is truly and absolutely secure, there is absolutely no amount of evidence of his inadequacy and mediocrity that can possibly convince him that he is not great and deserving of love from himself and from others. Such a person if they were in an abusive relationship would immediately recognize that he ‘doesn’t deserve’ that sort of treatment, on behalf of his greatness and specialness. An insecure person wouldn’t be so sure that they don’t ‘deserve’ abuse or hardship, and may even take the existence of the abusive person as evidence for that. In essence, to be truly secure is to absolutely impervious to any evidence of one’s inferiority or short-comings.
Of course, very few of us are truly secure. Generally we rely on a mixture of dogmatic narcissism(if we were raised by good parents) and societal feedback to gauge our self-love. Security can be described by having a strong core dogmatic narcissistic belief that you are great and deserving of love. It’s not a matter of being secure or not, but to what extent.
To conclude, being secure is having a solid base of narcissism from which to love oneself even in the face of feedback that utterly suggests one should hate himself. Having or not a solid basis for it is absolutely irrelevant.