I'm unable to continue working as a software developer due to an (as yet) undiagnosed mental deterioration. I'm concerned that I'm going to fall into a crack where I'm fired for being unable to work but not able to claim disability because it's hard to prove I can't work. Or maybe it will be blown off as "old age".

I've worked as a software developer for decades and have been with my employer (major tech firm) for more than 10 years. My ability to program declined slowly and then suddenly. I have an appt with my doctor but don't know what to do about work in the meantime.