A few years ago, I had an internship at a software company. For the first part of the internship, my performance was good, and I was making positive progress. At around the second half, there was a change in management.
At around the same time, I developed two problems that caused my performance to nosedive. The first problem I had was that I developed non-serious (diet-related) health problems that caused me to have a kind of 'brain fog', along with constant stomach problems -- it took me a while to recover from this. The second problem I had caused my working environment to fail in an undeterministic way -- it flummoxed almost everyone I asked in my team, including my mentor. The seniors I consulted about this were good at helping me track down the exact flaws, but were too busy to help to the extent that was needed at the time. I feel that my poor performance, along with bad communication (on my part) with my manager, caused my manager to become frustrated with me, and for that reason at the end of the period I was not rehired as a proper member of staff.
Afterwards, I took time off from work for a long period to improve my health, and to grow myself as a person through self-directed study. I have also learned a lot from stalking workplace SE. I strongly feel that these problems would not occur to me again, and that even if they were to occur, that I would be in a much better position to communicate about and deal with them professionally.
I am now considering applying to the same place. I have been told that it is likely they will not pay too much attention to my resume, but rather the people in the company I worked under. Most or all of the other people I worked with have since moved on.
How best can I communicate about my prior difficulties and my self improvement when reapplying, and how should I approach alleviating any concerns that my former manager has regarding my performance?