Wat should be done to this relationship....PLZ suggest?

I have a GF with1 yr old relationship but wid times i found her very serious and she is talking about marraige stuff,but deep inside i know that i cann't think of marraige wid this girl as i found her unfit for spending my life wid her....So how should i get away wid this problem or relationship.If i say her straighaway than she will got hurt and if not than even in latter times she will be hurt even more......Kindly suggest me wat should i do??