Does comparing yourself affect your confidence?

I was at Swiss Ball class this week in an effort to keep fit. I've been doing this class for nearly 2 years now and, although we change our exercises on a weekly basis, I've got to know most of the exercises over that time. I don't think it's an easy class: I'm huffing and puffing, and usually the day afterwards my core stomach muscles are aching from the work I've put in (and that makes me feel all virtuous and like I can eat cakes for the rest of the week :-D).