"Yeah, we're breakin' free"

Well, "breakin' free" isn't always a good thing. Yeah, I'm breaking free from old love. I'm breaking free from some less-than-happy-making habits. But I also seem to be unintentionally breaking free from some people I love a lot. I can't seem to make everyone else and myself happy all at the same time.
Let's say I'm best friends with someone, Person A. We spend all our time together, practically finish each other's sentences. A while later, Person B steps in. Person B is like a replica of myself. I can see me in everything Person B does and has done in her past. Wll, now I have two best friends, and it doesn't seem to be an issue.
Until...
Person B goes through this heart-wrenching crisis that I actually can't see myself in. I can't relate, so what can I possibly do but comfort her and just be there? When I'm not there, Person B is depressed, and I do not mind at all just being there. I know time will pass and she'll be able to stand on her own again.
Well, I was right. The crisis has passed and she doesn't need me as much as she did. So what do I do? I run back to try and make up for lost time with Person A, of course. But I run to her door, and as I approach, the door slams; the last thing I see before the glum mohogany of "too late" is her back turning on me. As the lock clicks into place, I can feel my heart breaking. She's taken a part of me with her.
I've been pounding on that door; listening for a click; waiting for the knob to turn; looking for any sign that she may want to let me back in.
But doors tend to make better doors... than windows. And I'm locked out.
What was I supposed to do? Just because she came into my life second doesn't mean I love Person B any less. Was I supposed to just let her deal with this on her own? Would that have been better for everyone important?