What's the nicest way to tell an agency recruiter I'm declining a job I've said I'll take right before the final offer?

After two days of consideration, and a recruiter asking me to be clear with him about if I was still in the process and would take the job so his reputation stayed sound with the client, I have decided I can't take it, and will be miserable. (I had told his colleague I was out after we had a heated exchange about my doubts, but this recruiter called me back and talked me back into it. I regret changing my mind.) The business is making their decision early next week, but I appear to be a frontrunner.

The honest reason is that I don't think my mental health is in the right spot to take on the stressful workload, odd hours, and seeming lack of vacation that no one has clearly explained. He's tried to allay my fears but I just don't feel right trusting that they won't alter the parameters once I'm hired.

So how can I best phrase this to him? Is the truth best? Do I come up with something more damning and last minute like a life emergency? I don't want this guy to be punished for my inability to confidently move forward.

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