I Need To Change Some Things

I have motivation problems these days. I've been a landscaper for 12 years and my latest employer is extremely ambitious. He has big plans for his company. And he busts his ass to make it work. And he makes it look so easy. I can understand that since this is the only income his family has. But I just can't keep up anymore. The heat is getting to me. The sprains are catching up with me. All the damage this job has been doing to me is making itself known. I fight it everyday. I constantly tell myself to keep going and to go faster. But I pay for it in the end. Because I'm so worried I'm doing something wrong again or I'm not moving fast enough and I let things slip. I've been forgetting tasks and basically pushing out information that I supposedly received that I simply have no memory of. It wouldn't be so bad normally but this time my employer is also my older brother. And it's almost everyday that I forgot something or that I'm going too slow. And I get it. It's his company. He doesn't want to pay someone to screw up and have to go back or to move slower than he thinks they should. And I can't handle disappointing him. I can't handle failing. Especially him. In my darkest of days he was one of the only people who didn't give up on me. And I want all the best for him. And I want to want to be here when he makes it. But frankly I don't think I want a career in this field anymore. And I hate disappointing him. So I need to change something. I just don't know what. I don't know the right answer.