When did you resign yourself to this, OP? What I mean is, when did you last stop applying for jobs that were above nmw?
Also, how old are you?
Have you looked at doing voluntary work (even if only a few hours a week) as a way of updating your skills and/or building your confidence?
Is it too late to apply to do the qualifying year?
I think if you really want to address this, there are ways to do it.
I have found myself in a similar situation and it did used to get me down, but I am slowly coming out of that situation now, and it matters much less than it used to because I have found a better paid job that I love. I'm still not earning massive amounts of money, but I have great job satisfaction.
When I was a SAHM, it didn't matter, as it's a viable and worthwhile career alternative, IMO, but once I needed and wanted to find work, then the feelings you've described really started to be an issue for me.
Like you, I would never think that about anyone else, but it was something I felt just about myself.
I am just not materialistic or money driven at all, which I think has been a bit of a factor for me, also, but at uni, I always thought I would have a successful career. That was my aim when I was younger, and I certainly had the brain power and qualifications for that not to be a realistic expectation.
Maybe try and do things that will build up your self esteem. Pursue a hobby which you would enjoy, but that means you have to interact with others, maybe? Good luck.