How do I know if I have too much work or if I'm incompetent?

My job consumes most every waking hour of my life, and--though I generally enjoy what I do--causes me immense amounts of stress. I want better balance, but I'm having trouble parsing whether this situation is my fault or the job's fault. Are my job responsibilities too much for anyone, or do I just need to learn to be a better worker? What tools do I use to figure this out? I started a new job in June at a rapidly growing company and was pseudo-promoted to a more leadership role in August out of necessity (project was dropped on my team). From about August to December I felt excited by the challenge and confident in how I was handling the rapid expansion of my responsibilities. For the past month or so, I have felt nothing but overwhelmed and near-paralyzed by stress. I'm still managing to function alright, but I really don't see this as sustainable. I feel like I don't have the context or the outside perspective to judge whether I simply have far too much to do, or whether I'm failing in some way (not working efficiently, prioritizing poorly, trying to do too much myself).

Help me think through the questions I should ask myself or others to gauge where the problems lie and how I can start to address them.

Complicating issues:
-I am the only person on this project in the whole company. I work in a team of three, but the other two have very little insight into what I actually do, and therefore can't give much feedback on how I'm doing, provide context, etc.
-The only other jobs I've had before this were at tiny (<5 person) startups or in academia, where things were either far more collaborative (more shared responsibility for outcomes), or far more structured (clearer role and expectations).