I get jealous when I shouldn't...advice please!?

Ok, so I've been going to the gym with my best friend for a couple of years, and in the past few months, a friend of hers from work has started joining us. I hate it, and I don't really know what to do about it. The girl is nice enough to me, I just get ridiculously jealous when we're there because they are kind of close. I know it's stupid, but I can't help it. I miss having that time alone with my best friend, and this girl is a lot more outgoing and fun than I am. Do you have any tips on getting over jealousy this jealousy? :( I hate it and it makes me feel like I'm being an awful person. I want to be friends with this girl, but I just get too worried that she's stealing my best friend. :(