Why does one small setback in my new role spiral into me thinking I'm a total failure?

Because the doom loop runs in your head, not in reality. Kris Kluver, in The Dysfunctional Family Office, captures Gail Mitchell's 6 a.m. office spiral after taking over as CEO. Brett went around her to Dad about the credit line, which means Brett doesn't respect her authority, which means maybe she's not ready, which means Dad was right to hesitate, which means Steve should have been CEO, which means she's failing, which means everyone will see she's not qualified, which means the business will suffer, which means she's letting everyone down. One staff issue becomes a complete self-doubt cascade in under a minute. The pattern is universal among new leaders, especially in family businesses where the original CEO is still around. The fix is to recognize the doom loop in real time and interrupt it. Naming it as a doom loop usually breaks it. Acting on it lets it run.