Should I quit my job or take leave (FMLA) because I'm having a difficult time trying to function normally.

Quitting your job because of depression is something that happens to people. If possible, I would try and be honest and open with human resources about what you're going through. They will try and help you work through. Often, you can go "on leave" for a while, without quitting. That way when you're feeling better or more capable, there is a job waiting for you. It's better for you and the company. Just my two cents. Either way, take care of yourself!