Is it normal to have a friendship like this?

Four years ago, I became friends with a girl who is five years older than me and has down syndrome. She's always fun to hang out with, but sometimes causes stress for me, and I'm not supposed to be stressed for medical reasons. Ever since I started my grade nine year, we constantly fought and we STILL do! The fighting has gotten to the point where my anger issues have gotten the better of me and was physical with her multiple times. But, I have been able to control my anger this week, and am proud with myself.

There are lots of times where I feel as though she isn't listening to me, and I have confronted her about it many times, and she replies
" I don't always have to listen to you!" When ever we fought in public, she would yell at me REALLY loudly so that EVERYONE can hear us fighting. She's done some uncomfortable things to me, and she's put me down when I tell her of what I wanna do when I'm older. We both wanna become professional singers, but she acts as if I'm taking it away from her! And when a fight happens, I feel as though I'm mostly the one that's getting pinned on for things, and she's this perfect angel that God had made especially to make people look bad. My friends have pointed this out and have asked me why don't I just end the friendship?

It is SOO complicated because she has down syndrome. It's SOO hard to ever say my side of the story, or say how I feel because everyone acts as if everything she says is right or ok, and I'm wrong and my feelings aren't important. I have even pointed out her flaws once and her mom got peed off at me because apparently; those things about her aren't true! What about me, what about my side of things, what about how I FEEL? I am stuck in this friendship that I can't get out of! Her mother won't let me!!!!!

Is it normal to have a friendship like this? Should I be THIS STRESSED in a friendship? She has also lied about my crush that I don't even know if the stories she tells me are true! Yes, she has down syndrome, I just think she's too smart not to realize her own faults and mistakes!