She seemed never to trust me. We are both 22.
Some of my best friends are married. But I haven't graduated, so I think I should have more time to prepare for my future. The day before she said she couldn't waste her time on me, she told me,"Why don't you understand how much I love you? You told me Amanda loved her husband very much, but do you know I would love my husband when I get married much more than her!" I thought she was out of mind at that moment. I had told her when she one day asked me whether we would get married some day, that I would not consider marriage within 3 years, because after all I am still young, and I am not sure enough she is my wife later. But I really like her. I wanted to give a time to see. But she didn't trust me. And because of that, I have suffered her "childish" manners for 6 months. I broke up with her.
However in my heart, I don't know whether I am right to leave her because I knew she devoted much heart into our relationship in the past.

First of all, I think you did the right thing in telling her how you felt about marriage. You're clearly not ready to even think about it, while she wants a commitment from you now. Does this mean you should have broken up with her? Well it depends on what you mean when you say "suffered her 'childish' manners." If you can elaborate more on what she did then I can give you my opinion.

If your not ready, your not ready. After only a year, that's to early for marriage, your just getting use to each other. I think she just wanted what her friends have, and it has nothing to do with you. You did the right thing by booting her impulsive self out of your life.