I recently started at a gym as a personal trainer, a month later I was promoted to the assistant fitness director, two weeks later (because of certain conditions) I was promoted again to the Fitness Director. Everyone has been really excited and happy for me except this one guy, he's been there longer than I have and obviously wanted the position but never brought in enough money to be considered.

Even though I'm now his boss, he's been pleasant to me but talking shit, grumbling to others about me getting promoted instead of him. Also trying to sell more than me to have more of a leg to stand on. I've only been there for two months and have always excelled past everyone, but this month we're neck and neck. How do I deal with this the right way with all things considered?