FMLA -- Family Medical Leave Act and ADA -- Americans with Disabilities Act are two separate and distinct programs, with different qualifications and guidelines to follow.
NEITHER is any sort of "job guarantee" or job insurance, however.
Both are applicable only to larger employers, so you need to find out if your company has enough employees to be covered by either or both programs.
FMLA time can be used for your personal health issues, but usually only after you've used up other sick leave and vacation, and a key here: you won't be paid while on Family Medical Leave, but your job should be protected. Just be sure you know what your company requires you to document while you're on it. If you end up using up all your vacation, sick leave AND the days allowed with FMLA, you can be dismissed. And if you're "vital" to your company, they'll find ways around the requirements and dismiss you if you're not there enough to do your job.
ADA won't apply to your situation unless you've disclosed to your employer (immediate supervisor, "human resources department", whatever other titles there are, out there) that you are disabled, and are asking for "reasonable accommodation". And it's the employee who has to suggest what that might be for the employee to do his/her job. If the employer agrees, and meets the accommodation, it's still up to the employee to perform the work to the same level of competence as before the accommodation, or the employee can be fired.
If you type in either of these programs into Google, you'll come up with their official websites that will spell out all the things you have to do to meet the requirements necessary to apply for these. You'd need to find out whether your company has enough employees to be covered, and then find out who in your company is the one who administers the program for your company.
People often think that the ADA especially is a "job guarantee for disabled people". It's not. If you really can't do the work well enough to suit your employer, *even with reasonable accommodation* '-- and again, YOU are the one who has to figure out what that might be, AND your employer has to agree to it -- then you can be fired. What YOU might think of as "reasonable accommodation" might not be "reasonable" to the boss, and if you can't negotiate the differences, you can be dismissed.
So before you go to talk with your employer, you should carefully read any employee manual that you might have now, read the web sites, ask questions, and seriously consider whether you really can do the job you're now doing. And if not, what might be a "reasonable accommodation". And if there aren't, then what....
...I am not a doctor, nor health professional, and don't pretend to be one, here.....