Why is my mom so annoying!?

I get in trouble for playing on my computer, I just lock myself up in my room. Then my mom whines that I'm on the computer too much. Then when I want to go outside, which IS often, she says 'no'. She thinks someone might 'grab me' I'm old enough to go outside, and hang out with my friends. But no. So now, I'm suffering from depression, because my family is screwed up, I can't hang out with my friends, and school is almost over, the only time I even kinda have fun, because I can't do ANYTHING while I'm at home. I sneak outside when she's gone, but come back in before she finds out. Though she is really starting to piss me off because she won't let me do a single thing. She probably just doesn't want to me to grow up, how do I get her to accept that I'm growing up and can't live my life inside, all cooped up. I hate it. They don't know I have depression, and I carry a pocket knife with me everywhere just in case anything were to happen, we live in a safe neighborhood. I just don't get it! Could some help me get her to accept me, or tell me what I don't understand....Thanks in advance.