Is it permissible to marry a religious and moral young man, but who suffers from psychological problems that led to his seeking treatment from a psychiatrist and taking medication, and who is characterized by extreme shyness and an inability to defend himself, along with fears of the impact of this on future marital life?

Do not rush into accepting marriage to this man. Instead, give yourself time to think, coupled with performing Istikhara. The psychological issues you mentioned affect the stability of the marital relationship, and you may not be able to bear their consequences. Do not wrong yourself or risk the future of your life and your family. For defects that are tolerated after marriage, it is not necessary to tolerate them at its beginning.