Employment Law Questions? Ask an Employment Lawyer.
Yes, what your employer is doing is a violation of the American's with Disabilities Act. They can send you to the doctor if they reasonably believe you are a threat to yourself or others, but they cannot insist you get further treatment when you have been assessed as able to return to work; that's a violation. You can/should fiile with the Equal Employment Opportunity Commission. The EEOC would investigate the workplace at no charge to you and make a determination of whether discrimination occurred. If the EEOC finds that, they attempt to work out a resolution or, if a resolution cannot be reached, they'll either sue the employer or allow you to sue them in federal court. Lastly, your employer cannot legally discipline or discharge you for filing with the EEOC. If this sounds like something you're interested in, you can learn how and where to file a charge with them here- https://www.eeoc.gov/employees/howtofile.cfm
If you resign, then you add the factor to your case of constructive discharge. In which case you have to additionally prove that no reasonable person would have stayed in the position, and you'd have to try to mitigate damages by looking for another job. Neither of these ruin your case, but make the case more complex.
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No, not in any way is it a voluntary resignation. You should consider litigation for certain.
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