My best guy friend practically ditched me for his girlfriend...?

I've been best friends with this guy since kindergarten, and he's like my brother. Well, he got a girlfriend about 4 months ago (they have liked each other for FOREVER), but this stuff has been happening since around September. He didn't even tell me they starting dating. I found out through one of my friends who isn't as close to him (at least I thought), and since then, I've felt like a total third wheel (especially when all 3 of us carpool, which is our parents "great" idea). He invites this girl everywhere-even places that are just my family and his-and very rarely goes to anything that she's not going to, almost including my birthday party. I don't like him, but I hate this feeling-like I'm SO happy for him, I really truly am, but I feel so alone and left out.