The person she loved did not love her. When she gave up loving him, he regretted it. But regret was of no use because the person was no longer there. She was no longer in this time and space.
She came to another time and space, and everything unfamiliar could not change her desire to protect him. Since she could no longer see him, she might as well protect the boy who looked like him! Originally, she just wanted to see his shadow in that boy, but as time went by, she found that his appearance had long since disappeared. All that was left was the boy who had always cared for her--Gokudera Hayato.