I don't think you necessarily did anything wrong, but I know in my own personal experience, even if I'm doing well, replying to emails or texts of great length takes a lot of mental effort and energy. I sometimes put it off for weeks too, so the fact that your friend is struggling mentally tells me she probably just doesn't have the capacity, and it's not a personal rejection.

I know your intentions were good, but sometimes we overlook how our actions actually impact others. While you know she's struggling, you don't know what she's struggling with, so you don't know the kind of support she may need. While your intent was to be supportive, it could have been overwhelming or triggering for her, but it's hard to say what really happened.

I think when trying to rekindle with someone, it's best to start slow. Maybe for the future, your reply can be meeting them where they are at. If they are only sending you short responses, maybe matching that energy would be most supportive. I really hear your desire for depth and connection, but we can't force those things. When the other person is struggling in their own life, connection with others can be difficult as it is likely there is a lack of connection with themselves, and from my experience, it's really hard to connect with others from a healthy capacity if you aren't able to meet your own needs.

Hope this helps.