Ugh. I feel for you. I too have worked a couple of jobs in my recent experience that I felt I just wasn't catching on to, but I knew that I would, and I did, just a little slower than TPTB would have preferred. As our society has headed screaming into an instant gratification mindset, everything and now, that is what the youngers are used to and unfortunately, demand. Not sure I have sage advice, but I would just try to convince your supervisor that you are catching on, you will redouble your efforts (that is if you want the job, you may not?), and try to give your best sales pitch of yourself. Finally, I'm a firm believer that what is to be will be. Not everything is meant to be. With employment being what it is right now, there is another job out there for you, if this one isn't the right one.
Try to remain positive, even if you have to 'fake it 'till you make it' (gawd). Hang in there kid!