Claire, i think you already know what to do.

The question is: what is harder for you – putting up with her moaning/negativity or detaching yourself to continue your progress? I have done something similar before and we drifted apart. Eventually, when she gained some maturity and stability, the clingy and negative loop got destroyed – it took another 2 years though. In the end, she did understand her negative state and how draining it had become. I dont regret it. Good friendships survive bumps over time.

It doesnt have to be the harsh way where you blatantly tell her. Perhaps let time run its course and you could slowly instill the idea into her head about how involved, busy you are. Please stop talking to her so much if you are really busy and dont want to. She will get the hint eventually. You cant force any sense into someone unless they are really ready to take it in and act on it.

Think about it.