I am 15 and in need of help and advice.?

I feel down almost all the time but I hide it. I can't hardly concentrate in school and feel like no one understands me. I feel like I am fat but everyone says I am skinny, I weigh 127 pounds and am 5 foot 4. I just feel weird and alone. I am happy some, but I always feel a little down. Here lately I have been thinking about how I miss my deadbeat dad that I cut out of my life and I miss his family that never trys to contact me. I cut him out of my life a year and a half ago. I havent seen or talked to him or that side of the family since. I have my mom and stepdad who love me and I love them, but I feel like something is missing. I just feel a little distant from them but they do not seem to notice that I am down. I can't talk to my mom about missing my dad because she thinks it was right to cut him out. I just hate that I won't see them for christmas. My stepdad is great, but I don't know, I just feel weird around him. Am I depressed or what? Please help. I am also spaced out, bored, and stressed alot but I don't let it show.