my girlfriend of 2 years dosn't see hope for our future

hi, i have been with my girlfriend 2 years and in the begining things were great we would talk about marraige and what our lives would be like together, she said that if i ever asked her to marry me she would say yes. however in past few months if i bring up the subject of marraige she avoids it or says im more grown up now and dont want to think about stuff like that, if i talk about the future she says i dont know if we'll be together that long, i tried making plans for christmas for us and she said you shouldn't plan that far ahead you never know what could happen between us.
any advice

This could be a mix of things, but what I would do if I were you - is stop focusing soo much on the future. Maybe it is scaring her off alittle and she just wants to enjoy the hear and now?
Try to just enjoy the every day stuff.

yeah thats true it just makes me think to much on her sudden change of attitude it leads me to doing stupid things like tryin to pretend i dont want a future with her

Its really tough because i am having the same scenario with my guy right now. We have been together 3 years in June, have shared some fantastic times together, been so loved up and yet after a few arguments in the last 2 weeks, one in particular where he stormed off and said he wanted to finish it, things have not been right and he has not come to see me. However, he has called my mobile in the past week and said he honestly doesnt know if we will be together again, but he promises to keep talking!!!! I have no idea if talking means about us getting together again, or talking for talking sake - very non-committal. I am asking what i should do so anyone with input i would apprecite it. I asked some guys about what he said and they both said hes being stupid and sulking lol.

In your case, like someone said dont go on about the future so much, talk about the here and now, you may be scaring her off but she probably does love you - give her some space and not be so clingy. Good luck.