My mother-in-law keeps nagging me and creates unnecessary drama frequently. She can get upset about anything. So I don't understand how to speak with her. I have lost my mental stability because of her.

It is very hard to speak to someone who gets upset easily. This kind of person may have a personality disorder, high needs for attention and a love of drama, which can make communicating with them difficult. The answer for you lies in not taking their drama personally and in making non-emotional responses to their outrage and nagging, like "Oh really, that must be hard for you" or "That's a hardship for you for sure!" or "Sure, that's a good suggestion, I am already comfortable with how I do it but thanks for the idea." Refuse to engage deeply with anything she screams about, let the nagging slide and have pity for her instead of feeling it's personal. She is likely insecure, needy and lonely and uses the negative communication techniques of nagging and outrage to get attention because she's never learned how to communicate assertively. Most of all, remember to never stoop to her level: keep your comments calm, neutral and objective, no matter how emotional she gets.