I feel suddenly mentally ill recently, and my mood is often high and low. I often desperately want to give up for a second, and I don't know why I suddenly burst out laughing.

According to your situation, you should be a patient with intermittent depression. This situation is often caused by external stimuli, not strong stimuli. It is caused by a long-term lack of self-comfort. I suggest that you'd better see a psychiatrist. Or, or. There's another way. You write down your inner activities with a pen and then observe them at a normal time.