I'm a fairly experienced software engineer also working for a startup. I think you should be forgiving with yourself. You sound skilled for what you're handling and your current experience level. You also sound overworked. This is such a common problem for startup engineers. It's very common to push yourself to very long hours that becomes unsustainable in the long run due to burn out. I think this problem has gotten worse since the pandemic. I've had to deal with burn out too. You need to take a break from the heavy hours at least a week or two where you completely unplug. None of this being on call 24/7, just completely unplug and don't read your emails. Focus on centering yourself, take a walk through nature or visit the beach and get out with some friends/family. It'll make a world of difference when you come back.