@Snowflake this time last year I was in your position. Overwhelmed at work, considering inpatient and terrified of losing my job. I submitted for FMLA, but only my HR department had access to the form with the information related to the reason for my leave. As protected information under HIPPA the HR department can't disclose the reason for your leave only that the leave has been approved. Legally, they can't fire you as long as you have time remaining on the 12 weeks of protected employment.
It is important however to be make sure that you understand your company's FMLA policy. They can make you use all of your accumulated sick, vacation, personal days and/or paid time off concurrently with the FMLA time off. In other words if you 5 days of sick time accrued and 10 days of vacation the first three weeks of your leave would be paid good for you, the other nine weeks unpaid unless you have a short term disability policy that covers your condition. Heads up some commercial STD polices through your employer will cover behavioral health issues but I have yet to find a consumer purchased policy that covers behavioral health issues. I was very lucky and my policy through my employer covered behavioral health so I was able to take the entire time off paid.
Another thing is that the 12 weeks that are covered are "rolling" meaning they do not follow the calendar year they are the 12 months from the date you are approved ie: December 1, 2018 through November 30, 2019. If you have other questions I can try to answer and I hope that this has been helpful.