Am I harming my skin by not using foundation everyday?

Lol no you're doing your skin loads of good by not wearing it. Foundation itself(no matter what it claims) harms your skin. It can clog your pores and give you premature aging(unless you use really really good ones, then the side effects will be less noticeable) If you cleanse your face twice everyday, and use moisturizer and sunblock, then your skin is very protected.