My really close friend is dating my ex that I still have really strong feelings for?

Well we have both been dating him off and on several times but this summer we decided to break up at least till school started cuz it wasn't working out because we couldn't see each other at all but then a few days before school starts he texts me telling me that they are going out, so when I ask her about it she says no and gets all offended. He has told my other friend they are so I'm pretty sure it's her lying to my face not him. I really don't know how to react to the situation and I really want him back but I can't make her mad because she is my partner in science for 18 weeks. And I can't avoid him because I have slot of cles with him and I am forced to sit at the same table and he always talks to me. I'm so frickin confused and every time I see either one of them I just want to start crying. Help please. Sorry it was so long!