Firstly, i would like to preface my reply, by commending you for broaching the issue of Mental Illness on this forum. I am sure like many other forum members who live with a Mental Illness, will find this topic of discussion quite illuminating, refreshing and invaluable.
Personally, i have suffered from Depression for all of my adult life, in varying degrees of severity and duration. During that time, I’ve maintained a steady relationship with the iron 
and have found amongst the positive net results it endows the diligent trainer i.e Improved body aesthetics and strength increases etc it can play a facilitatory role in staving off the ruinous effects of Mental Illness, to varying mitigating degrees.
The training experience is cathartic, even meditative in nature and affords the trainer qualities - that can be applied to everyday life with much success - such as structure, routine and the important habit of goal setting,therefore allowing you to redirect your energy and focus not on your internal turmoil, but something more extrinsically positive, attainable and productive.
In all reality, there are times when life circumstances trigger and exacerbate inherent symptoms, which distort and overwhelm your perception, mood, energy levels and behavior; effectively wreaking havoc on a psychosomatic level. Physical output in the form of any type of training is usually the last thing, that is of a requisite priority. It is still important though to engage in some sort of physical stimulus in this transitory low state - i know it’s bloody easier said than done, right!! - as it does provide a tempering effect.
When i am faced with the aforementioned situation, i usually refer to a form of training i call Default Training - a fancy term for deloading if you will - which is basically of short duration and moderately light in intensity. I find Neural Charge workouts are perfect for this type of training as long as you don’t overdo it, and, if possible performed outside in the fresh air, which i have found to be one of the most conducive aids when suffering a depressive episode.
In time when you feel you are in a better place and more balanced, then you can return to your normal training routine. At the risk of sounding cliched, with the right type of therapy, medication, education, exercise and support network, you can still live a deserved life full of quality. I apologise for my long winded and digressive rant. If there are any forum members who are currently suffering, please do not hesitate in contacting me, i am always read to listen. We are all here to help one another, as Mental Illness thrives on anonymity, denial and ignorance. God Bless and all the best for the New Year.