I didn't say she was a bad lady? I'm not saying to say it as an attack, I'm saying you need to clarify which version of imperfection is preferable to her, and fail on that side. She shouldn't be getting on you about 8 hours of work taking 8.5 sometimes, that's how the service industry always is.
You need to handle your stress better. A good way that ive found is to think of yourself like a rock in a stream when you feel stressed out. You are immovable, things just happen around you while you handle whatever work is needed of you. Full on defensive zen. You won't make forward progress when you're a rock, but you also won't lose any. Bring to mind whatever is true, and accept it as true. When you dont need to be a rock, you can work on changing that truth, but until then, it simply IS true.
Also, it's not that hard to become a teacher, if you have a bachelor's degree. And if you want to sub, there are zero qualifications afaik. Would be a good place to startits probably a better job than you have now.
You really need to get professional help, if that situation got you actively suicidal. Don't wait for another episode, at that point you won't want to reach out. That is a major overreaction to someone being a major asshole towards you. There will always be people who are shitty to you, but you can't give them that sort of power over you. You still control how you react to them.