Im in Love, But she Doesn't Love me. What can i do?

Me and this girl have known each other for three years and in that three years she has loved me but i rejected her and then i loved her but she rejected me then the year after she loved me but i didn't. She changed her group of friends and her personality but i still knew she was there and i love her so much. She got a boyfriend with bigger muscles than me and loves him. This all happened and now i love her and missed out. Im full of regret and wish i could go back to the gaps of time where i could have asked her out and we would be in love to this day and on. :( What should i do?