Reading through your responses, I'm not sure I can give you great advice because the two large software companies I've worked at welcomed moves out of what you call sustaining engineering. It was a pretty natural and repeatable process of fixing small bugs to fixing larger bugs to fixing bugs that impacted multiple areas of code. People would get assigned other people's regressions to fix. At that point, even if a sustaining dev manager doesn't want to let you go, the new dev team will damn sure want to have you.

If you really are terrible at programming, or maybe you just don't enjoy it very much, then explore something else. But if you do enjoy it, you have to figure out what's going wrong before you can make a plan or get good advice to fix it. You say people were dissatisfied with you as an engineer. Why? If you ask a more senior engineer or a manager what you can do to improve, you'll get a better answer than HN will be able to give you. But you have to be willing to hear some things that might not make you feel good. In the long run, though, it will be worth it.