Friends...but enemies?

I have this on-going problem with my best friend...er I guess my only friend, which only makes it worse. When we spend time together she constantly makes fun of me and treats me like she's better than me. I honestly don't know what to do. I mean, yeah, the logical thing would be to tell her how I feel, but I know she'll get pissed and I can't risk losing my only friend.
When we spend time with a larger group of friends (Mostly people I know from school) she does things to embarrass me and to make herself look good. She's stronger than me and likes to do stuff like put ice down my shirt and hold my arms so I can't take it out. She won't stop when I tell her to, and nobody else tries to stop her either. Is what she's doing considered acceptable to other people?
There are rare occasions where she won't do stuff like his and is actually nice, but like I said, rare. I just don't know why she does stuff like that, and it makes me feel horrible.