I know I'm good at my job as the leading government body for my profession graded me as outstanding. I also train others, my last trainee was the same age as my dad. I made him go back and re do stuff all the time. I just made sure I was respectful, Bu direct. I praised the positives and clearly highlighted the negatives. And when I reassessed if he hadn't made the improvements I pulled him up on it, but helped him with it. Reassessed again repeat etc etc. I had 6 months to train mine, you sadly don't have that time frame. If h's that bad, speak to your line manager.