I can't get over the fact that my ex-bestfriend is flirting with my crush. How can i Deal with this!?

This is what sucks about being your age. Teenage girls are mean to each other. REALLY mean. Firstly you have to remember that you'd be hingly jealous of ANY girl that flirts with that boy. It's going to sting, no doubt, but put it in perspective and forget about that part so you can focus on the part that's really doing you wrong... the bad mouthing. You need to find small opportunities to worm your way into his life, so that YOU can become the friend that is comfortable and welcomed to walk right up to him and talk to him at your leisure. YOU become the girl whose opinion he trusts. A new school year is a-starting, and unless you patch up your friendship, it's not going to be a fun one without you taking charge. Look for any possible opportunity to talk to him and show him that you really are a great girl, worthy of his friendship and trust. Then one day, you can tell him how you feel. Maybe by then (or perhaps already), he'll feel the same way you do.