Can someone still be “the one” if they never loved you the right way?

I’ve noticed that my GF often looks at other men when we’re out. I brought it up before, but she still does it just more discreetly now. Is this a red flag, or am I overthinking it?
My trust is ruined from what he did but I don’t want to leave him, what should I do?
Why am I feeling like my destiny is to be forever single? I'm 24 years old and never had a boyfriend. What should I do?