This is a very complicated relationship. You’re saying that it isn’t romantic, you don’t love her in that way… ok, so you two are just friends, like siblings. But a really close friend wouldn’t text you saying it’s the last time they’re texting/calling if there is no fight beforehand. If she doesn’t want you to help, then she doesn’t have to tell you about her problem but can still talk to you. It doesn’t make any sense. This is what I think is going on…
***** You can read this, but I didn’t read RV’s post before typing this. Apparently, she wrote the same thing. ** 
- She likes you in a romantic way and is doing this to either get over you/get you out of her life. She could even be playing a strange form of hard to get. She may be older than you, but older girls can fall for younger guys and vice versa. There’s always a chance of the development of some romantic feelings between close friends.
or…
- She’s currently in a romantic relationship and thinks your closeness is going to harm it. Maybe she has a possessive boyfriend.
or…
- Maybe you got it all wrong, she doesn’t think you’re that close of a friend. She might help you and understand you, but she just simply doesn’t want to be too attached for whatever reason… could be her personality or yours.
or…
- Her problems are very serious and if you get involved, you can be harmed in some way so she’s trying to protect you.
If you’re worried about her, then go to her house/school/workplace, wherever… but meet her so that you can find out why she’s avoiding you. If she flips out and tells you that she never wants to see you or talk to you, then just let her be and move on.