This is a very difficult situation Dan but I do think there's something important here about knowing what is and isn't yours to deal with.

If you were still in a relationship with this person, that would be different, but you're not and it seems you're still heavily invested in her life. I understand if you still care about her, but it sounds like you're both caught in a dynamic that you're unable to break free from, especially if she's turning up unexpectedly when you don't want contact.

Who your ex sees and how helpful that person is, is really for her to deal with now. I think the priority for you is to focus on maintaining your own boundaries with her. I know it's tempting to try and work out what is going on with your ex, maybe that's a way of unconsciously holding on somehow, but I think doing so may inadvertently keep her tied to you, as she will obviously know that you're continuously invested in her mental health.

If you want to let go, then let her go. And draw the boundaries around the relationship. It's harsh, but sometimes that's all we can do.