My first for real full-time software engineering job was for a very small company. I was self-taught and their only software developer was leaving in 3 months, but had mostly checkout out already. The company environment was find but all the software was terrible.

Over that first year there it was basically the Klingon Rite of Ascension. Fighting fires and trying to keep the company afloat. I had no backup/mentors and if I failed then the company was done. I was near tears (or privately at tears) at a few points from frustration and stress. In the end I think this experience made me a better person for it.

First I was forced to learn how to manage stress. This was a big one and one I had never mastered before. Next I need to be able to juggle and prioritize actual company needs, not just what people told me to do. I couldn't spend all my time fixing things when our customers also needed our product to work. I also handled much of the customer support at this point because most of the issues were software related.

In any case, in the end I pulled the company back from its instability. This gave me a good amount of confidence that I am a good engineer. That no matter what gets thrown at me I can fix any of our problems and develop new systems. Even if I need to outsource some of the work to people who are better at particular platforms.

This probably isn't a healthy way to overcome impostor syndrome, but its how I did.