Sorry you're not feeling well right now. Perna is right in that you need to apply for both SSDI and FERS disability retirement at the same time, but being approved for FERS does not depend on being approved for SSDI. Actually, it is easier to get FERS than SSDI, just based on the definitions involved.

With SSDI, you need to prove that you cannot work in ANY position available in the local economy. With FERS, you only need to prove that you can no longer perform YOUR CURRENT JOB, with or without accommodations. I've heard stories of people who had chemical sensitivities to something in the building they worked in and could not be reassigned to another building or in a new position at the same pay grade and they got FERS disability. But certainly they could work somewhere else if the substance they were sensitive to wasn't there. But that doesn't matter with FERS - it's job-specific.

I was approved for FERS disability last November, but I was already receiving SSDI (I had been on it before and was reinstated when I got sick again). If you've got the Federal time and Social Security work credits, I'd try for it. The way OPM figures out your monthly annuity is a bit complicated and involves different offsets of whatever SSDI you receive. It can take a long time for OPM to process the application, but if you're approved for SSDI, that will typically happen first. (But if you're not approved, that doesn't automatically disqualify you for FERS.) You can ask your agency to keep you in the system and not separate you until you receive a decision so you can keep your health insurance intact. The govt will pay your premiums while you're waiting but then you will need to pay them back out of whatever back pay you might be owed. They'll cover your life insurance as well. Dental and vision premiums, if you have them, need to be paid out of pocket through Benefeds. Your HR manager can connect you with them if you need to. You get to keep your health and life insurance into retirement as long as you've been covered for the last five years (or from whenever you were eligible if it's less than five years). You convert over to a regular retirement when you turn 62, with credit for the years you were disabled.

I highly recommend getting a lawyer. There are lawyers who can do both your SSDI and FERS applications. They know exactly what OPM and Social Security are looking for and can help you focus your applications. A good lawyer will also work with your doctor and supervisor to get the best supportive letters. Some work on a flat fee, some work on contingency or a mix of both. Mine was a mix of both - a small flat fee to get things going and then a percentage of my back pay. I'm not sure what the fee structure would look like with both FERS and SSDI being worked on (mine only did my FERS app), but if it means the difference between an approval and a denial with the headache of appeals, it's worth it to me.

Feel free to PM me if you'd like to talk about it some more.