Apologies for making yet another post about jobs, but it's beginning to feel like I'm completely unemployable.

Earlier this year I hoped I'd be able to get some work experience relevant to my engineering degree I'm working towards, and sent off emails to various companies asking if I could help out with CAD work...got some nice rejection emails, but nothing more. So eventually I gave up on that, and started thinking about working in administration...volunteered as an admin assistant to give me some experience, then sent off CVs to various jobs advertised online...heard nothing back except for the odd automated email thanking me for applying. Then I moved into the flat I'll be living in in term time, and as well as continuing to apply for admin jobs online I also went round shops looking for work...a lot of them wouldn't even take my CV because I can't work after the start of the semester, but the ones who did and said they'd be in touch...again, I've heard nothing.
I've given up searching for work this summer because there's not much time left, and am going to volunteer for the rest of the holiday instead, but what if this keeps on happening?

The only thing I'm good at is studying, and for the jobs I'm applying for nobody really cares. You can't learn work experience!
My sister's 16-year-old friend is working as a sales assistant in a well-known fashion chain, as her first job. I have two months' experience as a cashier in a supermarket (God knows how I managed to get that), and a month's experience volunteering as an admin assistant...I know it's not much, but I thought it'd be worth at least an interview! Just makes me feel completely worthless...what's wrong with me?