Why won't she share her problem with me?

Nick Jax Says:
Hello guys!! I have a 9 month relationship with a girl two years younger than me (I'm 19).Our relationship is tough and our different characters make it tougher.We live far from each other and we "lived" almost our whole relationship from a phone.But even if you can't understand it (it's natural...i could not understand also before this relationship) it's a relationship with passion and love from both sides.When i'm going back and think of our good and bad moments it's like it did not happen from a phone.Where the problem starts? the last few days she was "cold" and we did not really talk for long.I was asking her many times if anything is wrong but she was always saying that everything is ok.Last night, for another time i kept asking her and she finally told me.But it got me more confused.She said that for the last week or so she is bad...she's crying all the time, she's unhappy, she is closed to herself but she doesn't know the reason why! I asked her if something happened with her family, friends, an ex that she had feelings for, me...but she said it's none of these.She had no clue!What hurt me the most is that she said "it's my problem, i will solve it alone".I really want to help her because the way i see it her problems are my problems and i certainly feel like her...i don't feel good(and it won't change until it will be solved).That happened because (she said) all her exes did not really care about matters like this and she couldn't help but be "outlandish".It's the way she learned...she said she changed this for me but again it's not enough.She needs to understand that it's easier to solve it together! I suspected that it's because we had a quarrel some days before she started to feel like this.I was really mad at her plus many times she says that she is not what i deserve, she thinks that she hurts me more than making me happy but what i really think is that she's underderestimating herself...she loved me much much much more than any other girl!! But again she said it has nothing to do with the quarell!! What do you guys think? in your opinion what's the reason? and how do you think that she will stop being "outlandish" with me? (not because i want it but because SHE would want to change it) from my side what's the best thing to do??
Thanks in advance!!! 

There's not a lot you can do if she is not prepared to share the situation with you, and to be honest that doesn't bode well for your relationship - keeping things from your partner undermines mutual trust. Having said that she's a little young to be in a committed relationship and know herself and what she wants.

This leaves you with a stark choice - wait for her to get her head straight or move on. Though the age difference between you isn't great, you are of age and she is not. You are asking a lot of one so young to trust you with things that are troubling her, especially since you are still rather young yourself. The choice must be yours, but my honest opinion is that this girl is not ready for a serious relationship.