Disability start date question

I will turn 64 this year. I have medical conditions that limit my activities of daily living. I have been under treatment for these conditions for close to ten years. My 40 year career span has been in the new car industry and my pain from arthritis has severely limited my ability to perform the necessary job requirements. I was written up for poor performance in April of 2015 and subsequently terminated in January of 2017 for poor performance due to my inability to walk more than 80 steps or stand for more than 30 minutes. Would this written warning be considered the actual date of my inability to perform my job? I was unable to meet the required sales quotas during the subsequent months. My arthritis has been medically documented for almost ten years and has progressively worsened. I am being treated for an autoimmune type of arthritis.

I applied for early social security benefits last summer in an attempt to supplement my dwindling income because my goal was to continue working but, as above, I was fired because of poor performance, as I was written up for.

My question is this, is there any way to obtain a full disability payment instead of the reduced social security payment I am currently receiving. According to information I have found on line, if my disability occurred before I applied for social security benefits then I would be able to receive the full amount. Also, if my disability is found to begin in April of 2015, then what will happen to my payment when I reach my full retirement age? Does it remain the same or change?