Not having people there to rely on can make things incredibly difficult and frustrating. What is going on with your friends? Do you think that you could go to them and say, "Hey, I'm really struggling right now and I need some help." Maybe they just aren't aware of how you're feeling.
What about talking to your school counselor or a teacher about the grades issues? If you feel that your mom wouldn't understand, you could always mention it to someone at school and see how they can help you. Your teachers might be able to offer extra credit, or after/before school projects so that you can get some help boosting up your grades and have less stress to deal with.
It sounds like you feel you have to hide who you are. You shouldn't ever have to pretend to be someone you're not. Why do you feel you can't open up and be yourself around your family? It might be that people in your family aren't exactly like you, but that's okay. No one is the same; if we all acted the same it would be a really boring world, or family, to live in. Being different isn't a bad thing.
As for the cutting, it sounds like you're using this to cope with how you're feeling. The thing is, is that cutting isn't the best way to cope because you're hurting yourself. Maybe there's something else you could do to help ease the pain without hurting yourself: like journaling, listening to music, deep breathing, yoga, something like that. Let us know if any of these suggestions will work. We are here for you. Always. Hang in there.