Boy likes girl, girl likes boy, other girl likes same boy...?

I need help. I just got out of middle school last week and am going to be a freshman in the fall. And I have a problem. There's a girl named Ava, who I've been close friends with since 6th grade and I've been told by three different people she likes me more than a friend. I really don't like her that way, and she texts me obsessively, always wants to talk to me. I thought I could just let it slide, but she texts me non-stop practically 24/7. Its not that I don't like Ava, she's really nice, but she's dreadfully insecure and sensitive. I can't talk to her without thinking it over at least three times in my head. And now, I really like a girl named Sarah. The only person I've told was my friend, Sylvie. Sylvie and Sarah are close friends and Sylvie says Sarah directly admitted she liked me, but it was awkward because Ava also liked me. Sylvie of course took the liberty to tell Sarah I like her and tell me Sarah likes me. Neither Sarah, nor I have mentioned it, because of Ava. I don't know what to do, because I don't want to upset Ava, but I don't want to let Sarah go by without doing anything. I'm seeing them both on Tuesday, June 28th, and Thursday June 30th, and then I'm leaving for my grandparents and I won't see either of them until September. What do I do?