When your girlfriend tells you that she likes another guy, read the lines in between: "I don't like you anymore".
She knows that by telling you that she likes another guy, it will push you away or even lead to a breakup. It's just another way of telling you that her feelings have changed and she's moved on.
Furthermore, if you feel like she still wants to be with you, regardless of her feelings for another guy, it just means that she's keeping you around as he backup. If it doesn't work out with this new guy, she can always come crying back to you. Is that what you want? To be her safety net? If she ever comes back to you, who knows when she will find someone else and use you as her backup plan again.
Why put yourself through this? Accept her change of feelings. Remember the good times that you had, learn from this experience and move forward with your life. Don't go backwards.