Has your submission ever let you down?
Yes. There have been a time or two where I was unable to get over a mental hurdle and be what she wanted me to be. This happened when I had hard limits in place but having them made me feel inadequate as a sub. I would eventually drop those hard limits but I felt bad, inadequate, and inferior before doing so.
Have you ever been criticized for your submission?
I have. I have had many aspects of it criticized by others, most of whom felt my "version" of submission didn't mesh with their views of it. These people were content to throw stones and were not interested in talking about the differences and/or sources of our views. Usually this has been in regards to being "too submissive," seeing me as a doormat, or calling me misogynistic for how forced feminization is a part of my lifestyle.
I should add that I have been complimented on it more often than criticized.
Have you ever regretted being or feeling submissive in a moment or in a relationship?
A handful of times. Mostly in the context of having a very serious discussion about a topic that was important to vanilla life or had a heavy bearing on the relationship and they played the "but you're the sub" card. It is not a good feeling.
Have you ever looked back and realized you made a mistake and how did you handle your submission going forward from that?
Yes. I don't err frequently but I am not perfect. Generally when this happens I beat myself up inside and work like hell on improving and never making the same mistake twice. It helps me out quite a bit if punishment unburdens me of this guilt.