I think you are overthinking it trying to find so many reasons WHY. I usto do that as well.
My friend, the why is because she is a selfish addict. Nothing else. Please put this energy you are putting on HER back to YOU, and get some help for yourself. Like atalose said above, be working to prepare what will happen so you are strong enough to avoid that mess for when she comes running back. Not out of love for you, but because she has used up everyone else.
Hugs. I say this gently because I know you are hurting.