Personally I think telling her no is doing both of you a world of good.
Of course the only part I really care about is that I think it's doing YOU a world of good.
She really CAN find a way to take care of basic responsibilities and obligations. The VAST majority of adults do just that very thing every day of their life.
Your ex was used to having YOU to turn to. She was used to having YOU to bail her out of a situation that might have required a tricky, skillful resolution.
Time for her to figure this out on her own.
If she comes back, let's have it be because she has realized once again how much she loves you and wants to share your life - not because she misses her co-baby sitter.
Stay strong. Follow your gut - seems to me you've been pretty spot on lately.
Blessings,
Bill
"Don't tell me the sky is the limit when there are footprints on the moon."