For post concussion syndrome (concussion was 3.5 months ago), is it good to have a lot of mental activity/concentration even when you feel strained/symptoms? my psychiatrist says it is, but online advice says to rest & that activity prevents recovery?

It is difficult when you get different information from different sources. It is up to you to decide whether the advice of your psychiatrist or the Internet is more relevant to your care. If the internet can show studies with a control group, you may wish to discuss these with your doctor.

Post concussion syndrome lasts at most 4-6 weeks following the head injury. Continued cognitive or emotional issues raise concern of a traumatic brain injury. Mental activity and intellectual tasks might help you cope. A speech therapist may assist evaluating your capacities. Neuropsychological testing could be considered.