I don't know what to do

I'm fifteen and I think I'm depressed. A lot of people if I told them that would think I was overreacting to feeling sad or upset, but I've been depressed since seventh grade and I've noticed the difference. Back then, I started getting upset a lot more than I used to, and when I got upset I felt a lot worse than usual. I started to cut myself. However, I rarely did it--only when I was really upset--and besides that, I was pretty normal. Back then I was overreacting. However, for a little over a year--pretty much all of ninth grade and so far through tenth--things have felt differently. I don't mope around, and it's not like I'm never happy. But I get episodes where I get so upset; I start crying so hard I can't breathe, I cut myself, I feel guilty about everything and start thinking about everything that's wrong in my life, which only makes it worse. I want to kill myself a lot. Normally, I just lock myself in my closet and let myself be upset until I'm just too tired to even care. Sometimes I cut myself which helps me calm down. One time I got as far as checking the pills bottles in our house to see how many it would take for a fatal overdose, but then my grandma walked in a I just pretended I got a headache. It takes a lot to hold myself back from killing myself, but thinking about vary things can do the trick, like my best friend or the thought of no one ever reading my stories (I'm a writer). I feel alone. I have social issues where I can't talk to pretty much everyone except for a few people I'm comfortable with, and even with them I have to try. It's not like I don't want to talk to people; I just can't do it. I don't have people I can talk to. I have considered telling my best friend, but I'm afraid that it will either a) freak him out and things will stop being the way they are between us or b) he will tell someone even if I ask him not to and then things will stop being the way they are period. Probably both. I can't tell anyone in my family. My stepdad would think I was doing it for attention and my mom would freak out and never let be alone again or handle sharp objects. My life isn't that bad, although things aren't great. I have next to no friends because I am incapable of talking to people, my sister is perfect to my parents (she shoplifted and didn't get grounded, I used too much conditioner and got grounded for three weeks), I have bad issues with handing in homework and I've realized I hate myself way more than can be healthy. Basically, I feel this way and it's not going away like I thought it would. After ninth grade ended, it got a lot better, but for the past two months its been starting back up again. I've realized that I need to actually do something to get better, but I don't know what to do without telling someone and completely ruining the parts of my life that make me happy. What do I do?

[Althera]

Start with telling someone like you're telling us.  If there is an adult you can trust to not freak out and overreact, then they're the best choice.  Find someone or something worth fighting through this for.  Try making a list of the ten good things you noticed in the world that day.  Then twenty-five. Then thirty; keep going up until you feel better.  
If you ever need or want someone to talk to, message me? :)

[bryjay]

You don't have to suffer the way you are, everything I have just read explains my life perfectly, except one thing , I told my friend , not my best friend but a close and trustworthy one all the same , it makes life so much Easier, knowing that even if u are feeling down and like yo do want to cut , you can go and talk to them even if you just tell them that your not OK, you don't need to go into detail about what's wrong with you, though it would be best if you did, I know how you feel as I am suicidal also, but if u do not believe that ur life is worth living then at least think off this , the people left behind , please message me if you want help with anything, or if you just need someone to talk to , remember I am there for you =)