I don’t know what to do

So I’m 16. My parents don’t know I’m depressed. And I know I’m 16 oh I’m just sad right? But I’m telling you it’s not just sadness. I’ve been dealing with this for 4 years. Everyday I get worse. I’ve had days where my break downs are so severe that I got sick from crying and hyperventilating so badly. I have asthma so it’s really bad when I cry because I can’t breathe. I’ve been through rough times and yes I have cut. I’ve been clean for about a month but I still get the urge whenever a wave hits me. I don’t get happy anymore and all I feel is just dark emptiness. My real smile is just fake now but no one believes me. No one understands and I’m suffering everyday. It’s just gets harder and harder to deal with. I am suicidal I know it is stupid I guess but sometimes I just don’t see the way out. I’m trying to get better but I don’t know how. Can anyone help me? I don’t want to tell my parents because my middle sister has the same problem and they couldn’t handle it so I don’t want to add extra stress to their shoulders. What do I do? I’m breaking more than I thought I ever could every day just more and more.