In my experience, if you made a mistake that set someone off, and even if you apologized, they would never be the same again, if they accept your apology. I'm a little impulsive, and even though my intentions are good in my calculations, my behaviors weigh much more to others.

It's natural to be negative. Our minds are designed to be negative. Actually, depressed people have more accurate depiction of reality. It's that the non depressed people who have optimism bias. I'm negative in nature, but I'm trying to modulate my thoughts and behaviors to be positive as much as I can, because even though optimism is a bias, it also can be a self-fulfilling prophecy for a better life.

By the way, our brains are plastic even as adults. There is a book called The Brain That Changes Itself: Stories of Personal Triumph from the Frontiers of Brain Science by Norman Doidge that explains this.