Disclosure can definitely be a personal and complicated decision. There are potential pros and cons to both side of the issue, and it's important to know that in general, disclosure is not required unless you are requesting accommodation (although some jobs may require medical information to address potential safety risks).
There is one point I would like to emphasize, though. Employers do have a right to respond to performance problems, including terminating an employee due to poor performance. For this reason, it's important to make sure you are proactively addressing any performance issues, whether they are related to your MS or not. If they are related to your MS, it's important ot know that an employer is not required to "forgive" any problems or stop any disciplinary procedures if you disclose later on that the problems were due to your MS. Instead, they are required to consider accommodations to reduce those problems if they are covered by the ADA, if you request accommodation, and if the problems are a result of a disability.
Addressing performance problems does not necessarily mean you have to disclose and request an ADA accommodation. Some people are able to self-accommodate or find ways to fix the performance problems without requesting an ADA accommodation from their employer. It is your right to be able to request accommodation if you and your employer are covered by the ADA, but some people chose to self-accommodate so they don't have to disclose. (For an ADA accommodation, an employer has to be given enough information about your medical condition to understand it is an ADA request).
See previous posts here and the information on our website for more about your ADA rights and responsibilities:
Knowing Your Rights:
Finally, you are correct that it can take years for an SSDI application to be approved, so I can't emphasize enough the importance of financial planning if at all possible. You are not required to reduce your assets for SSDI (although SSI has income limits), but surviving financially through SSDI application process can take a toll on your finances. If you are thinking of applying for SSDI, I encourage you to contact us for resources and to review our financial planning brochure, Adapting, in advance so you can do as much as possible to prepare yourself in case you need it:
Best wishes to you,