This doesn't sound like true polyamory.

My best friend is going through a poly phase, yet when she finds "the one" there's no room for anyone else, she's devoted to that one person. She's afraid she's not getting everything she needs from one person, so she dates multiple people in hopes to fill a void she's not getting.

I hate to say this, but it doesn't sound like you are truly fulfilling her needs if she feels she needs other people to have her needs fulfilled. I think it's a temporary phase. If she loves you, she will come back to you, but I think at this point, instead of going through the motions of casual sex and false love because she's doing the same, look at yourself and what you're not giving her that makes her want to pursue other people and work on that.

Let me be clear, you are NOT a horrible person (and neither is she), but there is something going on that you need to work on to better yourself not just for her but for you as well. If you work on yourself, your relationship with her will be stronger for it.