I'm sorry that you feel like you have to give reasons why you weren't able to work 12 hours a day. That's an unreasonable, unhealthy, and frankly disrespectful expectation from your employer whether it's explicit or hidden. I'd encourage you not to try to justify or excuse it. I can't imagine that losing the job is easy, but

it's not your fault

that you were subjected to that, or that you did not match their terrible standards.