How do I get my "best friend" back?

You're young. You will more than likely meet up with her later, catch up and be friends again. In the mean time, it's hard, but you can stand. Talk to her more often. Don't push away her new friends because they "took her away". I did that with my best friend five years ago. We stopped talking for a year. Things worked out eventually, but it only upset her that I hated Kaylinn without even getting to know her. And yeah. Maybe this new friend of hers used to be horrible, but I'm friends with people I dispised in Elementary. I'm now a Senior in High School and most of the people I fought with and hated are my respected friends. You'll grow and you and your friend, Hannah will slip apart. But if you two were really that close, then that will always be in your heart and you two will always have a bond. I've been there, Hun. And I know it's lame, but you can always just talk to her more often, try to include yourself in their group. Make a trio instead of a duo. And I really hope things will work out. =]