Im in pain

I feel like I'm in complete despair. It was so sudden that everything changed. My girlfriend, with whom we've been together for 5 years, now broke up with me. I never knew that she already had a problem with our relationship. I always ask her, but she will just tell me maybe some other time, and I always just let her take her time whenever she is ready to talk about it. She kept the problem to herself for a very long time. She said she couldn't tell me about it because she felt like I would not understand her problem. Maybe I won't, but I am willing to listen and still try to comprehend. Our world became different. She had problems with work, and maybe that is what I can't understand. I'm just a student; I don't yet have any experience working in real life but have tried. A month before she broke up with me, she met a guy. She shared her problems with him and said that she felt like it was easier to open up to him than to me. As time went by, their feelings grew. When she was breaking up with me, she told me that she didn't love me anymore, as she used to, and she said that what they had with the other guy felt true. How can I accept that? How am I supposed to fix myself after seeing her fall in love with another guy? Why didn't she give us a chance to fix it? We never got the chance to talk about it together until she broke up with me. I may sometimes be immature and couldn't understand her problem, but I am here, willing to help. I know, right now, that there are only little things and a little help I can give her. I may be weaker than her, and she may often help more than I could do for her, but now I am decided and ready to step up. She still couldn't give me a chance to prove it. 5 years, and my love for her never changed. Why did she change?