Can I really force myself to focus?

Ok, so all throughout my life I've had problems with completing school work ignments. Normally each year I get good grades 1st semester then 2nd semester my grades just drop leaving me at the last portion of the year trying to get my grade back up. Each year It seems to get worse. Even more missing ignments, worse grades, and more punishments as a result. This summer I was diagnosed with ADHD, which explained SOOO much. I have a summer reading ignment this summer that I must complete a 140something question study guide and an essay that must be completed by this Thursday to turn in.(school started today) I'm at about 24 questions remaining on the study guide and still need to complete the essay. My ADHD medication recently became ineffective due to the tolerance that has built up with repeated use. My parents keep asking me why I can't just force myself to get the work done. Before I would have no excuse. When work hadn't gotten done before I was diagnosed my parents said I was being lazy and wasn't putting effort into schoolwork. Because my medicine isn't working right now it's a lot harder to focus and my parents are angry. They ask me, "Why can't you just force yourself to focus? You completed ignments before you were diagnosed. You can't just make excuses out of this(referring to my ADHD)." I think my parents have forgotten the decrease in my grades the past 3 years. I've been reading articles on ADHD explaining that it's okay and saying it's nearly impossible for an ADHD personality to focus in a schooling environment without treatment. Can I really force myself to focus with ADHD? I don't think my parents completely understand ADHD.