Depression

I posted earlier about feeling depressed, I was told it was just a phase and it was normal for my age at 13. It makes it so much worse when people say that you can't feel pain or be depressed when you're young. Why does it matter that I'm 'still developing"? My brother is 11 and has been diagnosed with ADHD and anxiety. I've felt worthless, unworthy of freinds and happiness, lonely, afraid, hopeless, and many other things I can't put into words. It feels like someone is in my head, telling me these things, and I argue back saying that I'm okay, but that person is winning. I haven't been myself, it's hard for me to feel or pretend to be happy. I don't have the strength or motivation to do anything. I've never felt so terrible in my entire life. I'm not looking for attention. I'd do just about anything to just feel okay. I'll be honest here, I feel like people will think I'm looking for attention when I say this, but I feel like the only way to cope is pain. I wanted to self harm today. I dug my nails as hard as I could only my hand and I carved words into my arm and legs with a pencil so hard it swelled up, it's been 2 hours now and you can still see the words and nail marks. I wanted to cut myself but I didn't want to leave scars or have people know what was going on, and I was kind of afraid. I'm kind of at a loss on what to do. I feel like a burden on my mom, especially if I tell her, and I'm too afraid anyways. I hate being judged, I feel like I can't trust anyone because they'll laugh at me and think I'm pathetic. I feel like such an attention seeker, and I'm not, I just needed a place to vent. I have no idea what to do, I know something is wrong with me but I don't know who to tell. I would tell my parents but I'm too afraid. I'm just afraid of what I'd do to myself if it got any worse. I know I need help, but I'm not willing to tell anyone. I've been feeling like this for around a week and it was particularly bad today. That was my rant. Thanks if you cared enough to read it all. 
EDIT: I have thought about telling my mom, I can't deal with this alone. How can I if I'm afraid to talk to her face to face? She's one to tell me I'm overreacting over everything. If I was to say I think I have depression she'd probably say I was overreacting and I'm fine, when I'm not. Thanks for any advice.