@Rahatr That sounds awful. What you describe is more about having abusively pushy parents (sadly some parents are just abusively pushy and the 11+ really does bring out the worst in some) than the exams and preparation. There is no need, no need at all to treat a child as your parents and teachers clearly treated you.
My DC all did 11+ prep and were successful at obtaining places at their desired schools (private, we did not apply to grammars). DS2 and 3 currently at a v academic SW London indie and really happy. I wouldn't say 11+ impacted negatively on their mental health at all. But we took a low key approach, one hour of tutoring per week, lots of positive praise and small amounts of daily prep (ie 10 minutes a day) in the run up to the exams. We never made them feel like it was the end of the world if they didn't get in.
My theory is that if you have to do ridiculous amounts of tutoring and preparation to get a child into a school, then they're not going to cope once they're there anyway! So whilst a sensible amount of preparation is essential, there has to be a balance.
I would say with each of our DC they had moments of being frustrated, moments of feeling anxious about the exams, occasionally getting upset if they struggled with a particular set of prep questions. Which is normal I would say for anyone studying for anything. But it was never to extremes. And all the schools we applied to made the assessment days as low key and welcoming as they could, which helped. They mainly came out enthusing about the food 
Of course all children are different, and some will by nature find exams more stressful than others. But if a child's mental health is being 'wrecked' by the 11+ then that's probably because of their parents, not the process.
I'm sorry you had to go through what you did.