Not to beat the dead horse here, but discussing the MCAT on your app at all can only make you look ridiculous at best. If taking the MCAT is truly the biggest stress of your life, then you're beyond lucky. And I for one would like to exchange lives with you. If taking the MCAT is truly the most significant achievement of your life, then I feel very sad for you. I came darn close to acing the MCAT, and I never regarded that as the most significant achievement of my life.
Like cabin and DrB have said, forget about the MCAT. Forget about any academic anything. What *personal* challenges have you overcome? What *personal* goals have you achieved? If you're 30ish years old, there should be multiple life events you could discuss in response to those questions. When I think of life's challenges, I'm thinking of real challenges. Death of a loved one. Loss of a job. Divorce. Raising children. Being kicked out of your house at age 16. Serving in the military. When I think of significant achievements, I'm thinking of real achievements. Sports accomplishments. Charity work. Civic or church activities. Accomplishing a goal in the workplace. Helping others reach their goals, especially children. Meeting a personal milestone.
You know what I wrote about in response to questions like these? Co-founding the MCAT subforum for SDN. It was a labor of love that still results in random people PMing me every now and then to let me know how much it helped them, even though I mostly stopped giving MCAT advice 5+ years ago. It also led to challenges such as helping to formulate SDN's exam discussion policies in response to AAMC actions against our members, and being involved in the tough decision to let one of my junior mods go. SDN is a microcosm of many of life's challenges and achievements. In one capacity or another, I probably discussed my experiences with volunteering for this site on every secondary from every medical school where I applied.