Why am I like this, can someone help?

Ok, I'm the oldest and im 17, I'm also a junior in high school. I have a lot of problems. I have mild asthma, mild scoliosis, and mild ADD. I'm also not good at anything such as academics and sports(except for fighting and pool). I'm the only dumb one in the family. My parents got scholarships to universities and my younger brother and sister are very smart and have done better than me when i was in their grade(s). I'm failing badly in school , i dont get girls, and i just think i'm a natural loser. But I have mental toughness and I'm able to get through anything, I also have natural leadership but no one ever bothers to listen to me. My main dream is to be a US Army Ranger. But I lack common sense, I'm clumsy, and I'm slow academically.So how come I'm like this? Can somebody please help me?