If a person of sound mind wishes to end their life, they should be able to. Forcing someone who is experiencing constant excruciating pain to endure that pain until the natural end of their life is cruel and inhumane.
First of all, the entire field of psychiatry is very dubious. It's nothing similar to a mature science. They aren't really doing objective evaluations. For example, very often, if you really want to take a medication, you almost certainly can find someone who'd write you a prescription for it. [1]
Also, why would someone's academic qualifications entitle them to essentially sentence a suicidal person to death?