What can I do about this?!?

Okay, so I had this BEST FRIEND EVER! He was a guy and I was a girl, but there wasn't any romance between us. But it's like we were destined to be best friends. He has this sarcastic sense of humor, and I have this weird one, so we'd always crack jokes about each other. We spent Pre-K until 5th grade together. (7 years) Then for 6th grade, he'd be going to a different school. I was really sad, and cried myself to sleep. After all, he was always at my side, and if I was nervous and told him, he'd make a really sarcastic joke that would seem rude to others, but I know he meant it kind-heartedly to me. Anyway, I always told him about boys I liked, to which he groaned, and we shared everything, and would have playdates every weekend. Anyway, 6th grade came, and it was OK. He and I still kept in touch, but were too busy during the school year, so during the summer, he came over about once or twice a week. I know the summer's nearing but I just feel so alone! Now I'm in 7th grade and I was hoping for him to come in the summer again. But once during the winter, we were at a friends house with about 3 other friends who also used to go to my school, and I was friends with, but now go to the same school as my best friend. So we were having a snowball fight, and everyone was ganging up on me. At first he didn't say anything, and finally when I thought he would tell everyone to cut it out, he joined in! This didn't seem like his usual self. I know he's sarcastic, but this was going far. They weren't trying to bully me but it sure felt like it! The even teams we had made for a snowball fight had now turned into everyone vs. me. They were pelting me with snowballs, stuffing snow down my shirt and making me lay face-down in the snow. Then, they stole my hat and dug a hole and hid it. I was close to tears. My hero ended up not being my best friend, but his little sister, who is 4 years younger than us! She found my hat and ended up hanging out with me for the remaining time. I havn't spent any "quality time" since with him. I figured it was just him trying to "impress" his friends somehow, but it still hurts to this day. Not that moment, but that I might have lost my best friend. I rely on him because at my school I have a nice group of friends, but none of them "know me" the way he does. He is actually the only kid from my school within my entire life that has ever been to my house. I feel lost without him, and meanwhile whenever I catch up, he keeps on talking about his great life at a new school with all these new kids. I know that I have to understand that he has new friends, but I want my best friend back, and I want to start living happily. I'm close to tears... please help me. How on earth do I get my best friend back?! :'(