How do I know when it’s time to quit?

I’m 32 and about to change careers. Or I suppose I'm going back to the career that I originally was on before my life took a side step. What’s strange is that just a few weeks ago I would have told you I was happy. I think I had gotten pretty good at convincing myself I was. All the signs and red flags that showed me my position was turning into something different were missed or explained away by my need to succeed in whatever I do. Admitting to myself that I can’t do this position anymore feels like a failure, and I hate failing.