Losing my Best Friend

D and I have been friends since we were 11 and best friends since we were 17. I'm 23, and it feels like we've known each other for a lot longer. Once we even bought each other the exact same thing for Christmas. When I lived on campus and had no Internet we wrote each other actual letters and sent silly things through the post, I still have everything she ever sent me. We used to go on ridiculous drives in her car, which usually involved us getting lost or stuck on the ring road, going round and round. We shopped, we lunched, we Starbucks'd, we ate our body weight in Nandos, we watched movies, we got drunk, we ate an absurd amount of ice cream, we wandered round town centres, we went to the cinema, we went to the theatre and almost froze to death (we were not adequately dressed for December). Endless private jokes, that sound like nonsense to everyone else. There are thousands of memories rattling around in my brain. Then, last year it all changed.
D met her boyfriend and started her new job, it changed her, took her over. She came to see me after I had my wisdom teeth out and was whacked out on painkillers. All she talked about was her trip to Dubai, with the boyfriend (I've never met him) and showed me literally dozens of photos on her phone. I barely remember it because of the interesting selection of meds I was on (everything was fuzzy).
My birthday rolled around, we hadn't spoken as much as we used to, but I figured that was normal, we were busy people. I threw a party, my parents were out of the country and my sister is highly bribable. A bunch of friends came, she was too busy seeing the boyfriend.
I have to point out, I wasn't jealous, and I don't resent her happiness, not one bit. But it was my birthday. One day of the whole frigging year.
She turned up about an hour after it started, he sat outside in the car, would have been nice if he'd come in and said hello. She couldn't stay, they were just passing through, to give me a card from both of them (bearing in mind, I have never met him and he didn't come in) and a box of chocolates that hadn't even been wrapped. (We are both big present givers, in the past the gifts have always been perfect).
Then she was gone.
I sent her a text to invite her to Sunday lunch a week later, she was busy with her boyfriend.
November: I was on a teaching course (learning how to teach English as a foreign language), she was also doing training for her job. I wrote her a quick facebook wall post, just to check in.
The response was unexpected to say the least. She private messaged me to tell me to never mention what she does for a living on her wall, anybody could read it. I sent back a message pointing out that all she had to do was change her privacy settings (her job isn't illegal or dodgy, the opposite really).
That was pretty much the last time we communicated.
Til this week. I was at another close friend's birthday party and I mentioned her. This friend asked me why I had referred to her as my best friend. It's an automatic response. D is someone I love and care about, an extension of my family, the person I trust the most.
So I thought, I'd reach out, see if our relationship was salvageable. I couldn't find her on facebook. I assumed she'd left it or something. No. Through her sister (who I am still in contact with) I discover she's still on facebook, we still have a bunch of friends in common. However she's disconnected from me.
I sent a friend request. I'm not crossing my fingers. I have no idea what has brought this about. It's breaking my heart. I'm pondering emailing her and asking what's going on.
I know she doesn't read this. Very few people do. Maybe the virtually psychic link we've had all these years is still working and she'll somehow know I miss her. That it's horrid. That I'm sorry if I didn't try hard enough to keep our friendship going. That I'd just like to know how she is.