Is it possible that I have ADHD?

My mom was diagnosed with inattentive ADHD as and adult and my biological grandfather as well. My cousin was diagnosed with hyperactive ADHD when he was really young, so its on both sides of my family. Ever since kindergarten my teachers thought I was spacey and a daydreamer. I worked really hard at getting good grades, and up to a point I succeeded. In 6th grade my GPA was a 3.7, then it steadily went down to a 3.6, 3.4, 3.2, 3.3, and in freshman year it plummeted to a 2.4. I'm now going to summer school because I failed the second semester of Algebra 1 (I took honors Algebra in 8th grade early,failed it,and retook it in 9th grade) I tried really hard to finish all my work but things would always distract me,like someone mowing their lawn outside or the birds chirping,or the T.V. going on in the other room so I would sit there doing one ignment for 4 to 5 hours. I always like to doodle on the side of a paper and tap my pen on the desk.I'm really quiet and I don't like to stand out in case I mess up. I've always been really messy and unorganized, my mom would constantly get on me for that. When I talk or tell a story I stutter a lot or talk slowly because the words I want to say come out all out of order, or my kind is halfway on something else. In 5th grade my mom had me go in for behavioral therapy,but they didn't see anything wrong with me seeing as my work load wasn't all that hard then. It started getting worse at the end of 8th grade and all of 9th grade. My mom actually sent in the papers from the therapy I had back then to my current doctor and is making an appointment. I just want this worked out before I return to school for my sopre year,so any suggestions?It's all greatly appreciated.