Should I include information about my mental health in my masters personal statements?
You should not. But if you write it in the way you wrote the question explanation (rich in useless details) nobody would consider it. Another aspect you need to work on: given the text, the only thing important to you are/were the grades, not career development and knowledge. The major reason you attend classes is to acquire knowledge and wisdom, with grades being a metric of how this acquisition went. If you go for grades without actually acquiring knowledge, your career will get very dull.

The statement is about your passion to the field (which is ... ?), about how you want to teach, sing, dance, engineer, heal, fight, prosecute or whatever the degree is about, how enthusiastic you are about it. While pushing this concept can be enforced by your past, the statement shouldn't be a judgement of what you did in the past.

We all have ups and downs. Finding the one responsible for downs (like the shitty people from the junior year for example) is an evidence of lack of personal responsibility. You had your down, you overcame it, that's good for you, but it was because of you, not because of other people.