My apologies if I took things a bit off course.
In the MegaTrav Referee's Sourcebook there's a tech level table that talks, among other things, about advances in medical technology. Relevant to this topic are the following:
TL11: nerve refusion, artificial eyes. So they can manipulate nerves with a good degree of success. Potentially implies some early potential for repairing the neurological causes of mental illness. This - or perhaps TL12 - should mark the end of seizure disorders and many mental disorders rooted in physical injury, anoxia, stroke and other insults to the brain, but other disorders may be too subtle to be reached by this kind of treatment. Also, repairing the brain doesn't unlearn what's been learned - the person would still have quite a bit of rehab ahead of him and may continue to have areas of weakness like social skills deficits.
TL13: Cloned body parts, reanimation. The latter implies a fairly significant degree of skill at repairing brain injury, since reanimation is not possible otherwise. I would say most if not all mental illnesses would be curable at this point.
TL14: Genetic engineering, memory erasure. The former implies an ability to identify and edit out the genetic causes of mental illness. The latter implies - well, I have to assume they mean memory erasure and you end up with someone with some cognitive function, or else it's just a fancy word for execution. Anyway, a rather radical but effective method of dealing with the kind of disorders that lead to serial killers and such: brainwipe 'em and retrain.
TL14 also introduces, under Computers, brain implants. if you can feed data to the thinking centers of the brain in response to signals from the brain, then it's pretty easy to introduce implants that do things to other parts of the brain: suppress undesirable emotions like rage or fear, stimulate your pleasure centers in response to specific desired thoughts and behavior, stimulate aversive feelings in response to specific unwanted thoughts and behavior. Actually, there's potential for that end of things to get downright horrifying, but it definitely has significant therapeutic potential.
Between the repair abilities implied by TL13 reanimation, the preventive abilities implied by TL14 genetic engineering, and the rehabilitation potential implicit in brain implants, mental illness at this point should be almost unheard of - assuming either you can afford it or there's someone willing to pay for it. One wonders how many gung-ho Marine recruits are carefully rehabbed psychopaths paying off their literal debt to society.
TL16: Brain transplants, crude memory transfer. The former isn't technically curing - more like killing the person and making his body available to someone else, but it might be a (somewhat horrific but effective) way to deal with brain disorder identified in pregnancy or early infancy: clone a healthy brain, transplant the healthy brain in place of the injured brain. Doom for the wee one, but the parents leave the hospital with a healthy "twin" of the unfortunate kiddie.
The problem here is being able to access and afford the care, which means a lot depends on the society itself. Some crowded high-law worlds might find it more efficient to practice euthanasia of the mentally ill, irrespective of tech level - or they may only offer the cure to those willing to accept certain attached strings (such as the aforementioned Marine enlistment). And, if you're living on a TL6 world, the fact that Rhylanor has a cure may do you no good whatsoever (unless, of course, Imperial recruiters are specifically recruiting from psych wards and maximum security prisons).
Fortunately, memory implants are a few tech levels farther on, or you could find that your soldier character's backstory was a carefully crafted lie implanted into his brain after he was wiped and rewired.