How can I get my best friend to trust me more?

So ever since we were both in Elementary School my best friend has never had any friends. I remember I always used to watch her sit in another part of the playground alone and watch everyone else play. The only thing that would go near her were nearby animals like birds, bugs and stray cats. Even I was to nervous to get near her, everyone thought she was dangerous because she never spoke a word. I remember as years went by people befriended her but always left her behind and betrayed her. I noticed because of this shes so reluctant to trust people. Now were both in High School and I managed to befriend her but she still doesn't seem to trust me all that much. How can I get her to trust me more?