My 34 M girlfriend 32 F of 12 years said no when I proposed to her. what I do?

My girlfriend and I started dating when she was 20 and I was 22. Despite having been a couple for many years, we do not live together, I spend a lot of time in her apartment and sleep there almost all the time. She mentioned marriage after two years we started dating but then she stopped.

A week ago I proposed to her, bought her a ring and made her a romantic dinner, but she said she didn't want to marry me. That she preferred our relationship to continue as it was before.

I'm almost 35, and I want to marry her, online together and start a family but now I don't know what her plans really are. I don't really know if I should continue the relationship or just break up. It hurts me, but I really love her and I don't know what to do in this situation.

What would be the best way to approach this delicate situation with my girlfriend, considering our differences about marriage and our future plans together?