What crazybacon says is truth, you are indeed a "fall back option" "hmmm, I reallly wanted it work out with that hot guy, I'm depressed now, I feel like I want to be needed. Let's see who was chasing after me a month ago! Oh there it is, I remember this sucker was all over me, I need to feel good about myself, let's message him"
That's her train of thought, right before she messaged you. So act accordingly, keep your guard up. My advice: don't mention or call her out on disappearing on you, not in a funny way, not in anyway.
Best way to handle this, is give her the minimal attention. As if you forgot who she was, and she's someone totally new to you...
Keep your responses short and generic. "Hey what's up!" "Oh yeah, that's cool" "nice..." act as if every time you're answering to her you're thinking to yourself "who the f are you agan?! How do I know you?" Let her get conversational, not you. Don't chase her, you also don't have to block/delete her.