I'm losing my best friend

Last semester, one of my best friends told me he liked me, and that he wanted to be with me. I didn't want to jeopardize our friendship, and I told him I'd been there before and that it hadn't ended well. He said it was fine, and we both promised things wouldn't change. For a while, they didn't. We hung out after he told me... just like always, no awkwardness. Winter break started the next day, and we wouldn't see each other for a while-- but we texted/fb chatted everyday like we always do. 2.5ish weeks later, we're back at school, and I find out he started dating some girl. He didn't tell me about it-- that hurt in and of itself. But now, I can't act the same way around him, and he doesn't either around me... things are clearly tense even though both of us are pretending they're not. The thing that hurts me most though, is that he hasn't been around at all. He stays over at her place much of the time, and during the day, we have class and all-- I've barely spent any time with him since we got back to school. Last semester, we'd always eat together, and spend nights either watching movies, playing poker, or studying at the library till late... we pulled so many all nighters together just hanging out (with some other friends too). Now, I feel like I can't talk to him anymore about certain things-- or at all. I've been so sad these past couple of days, and I miss him like crazy. At the same time, I'm angry at him, and perhaps that's not even justified. Although I feel like I made the right decision when I said no to him, I keep wondering, "What if?" After all, he understands me so well, and I could talk to him about almost anything. I don't know how to get through this semester..he's one of two people here that I'm able to connect with.  I'm definitely losing him...things have already changed so much. Advice anyone?