After confessing to a girl, she was rejected. I asked her if it was possible for us in the future. What do you mean she's not sure?

This is a shirking. Since she said she wasn't sure, she didn't deny it or confirm it. She may like you a little, but she doesn't want to accept you. She said she wasn't sure what she would leave you with. Anyway, I think it's always like this. Don't always love this person. Sometimes letting go may bring greater happiness.