I'm Having So Much Stress And Depression In My Life

I'm having so much stress and depression in my life and at the vary moment I still am. I just need some help, I don't know what to do. I'm only 13 and my parents don't know that I'm doing this. I know that I sound like that everyday girl wanting attention but....I really don't. My parents are divorced and parted in different houses now. My dad cheated on my mom with a younger girl close to my sisters age...18. And my dad is currently 50. My sister has grew uncomfortable visiting him now and not seeing him anymore, I feel the same way as my sister feels but I don't know how to tell him. It's hard going back and forth. My dad and I get into fights all the time, I just don't feel comfortable if you know what I mean. I don't even give him hugs or say ''I love you.'' I almost ran away to my moms one time. I have to be at my dads more often as i do at my moms too. And my mom does know how I feel about going to my dads. And I'm struggling in middle school now and I can't understand things as well as other students do. I feel a lot of depression and pain and no one see's it because I hide it all. It hurts inside and I constantly cry secretly I hate to admit. My dad Is scary when he's angry...he makes me cry also. I need help please.