I am sorry to hear that you are struggling with your relationship.
SAD syndrome can affect people in many ways and have a knock on effect on the people around them.
I am wondering whether your partner's significant and challenging request may reflect his own state of mind and perhaps a need to find ways to deal with his symptoms.
There are plenty of things that can be done to address the symptoms of SAD if he wishes to look into it.
It may help to ask yourself what you need to do to best take care of yourself and your children in this situation?
I would question how much your well-being is being taken into consideration here by someone who is asking you to leave your children?
I do hope you find the right answers and that if you need further support you could try seeing a counsellor?
If not NLP or Life coaching are other techniques used to help individuals tap into their own inner resources and find new ways to overcome challenging situations.
Both are helpful as they work to empower the individual which in turn can boost self confidence.
When looking for a practitioner I would recommend an online search for 'NLP or coaching accreditation bodies' to ensure professional training standards have been achieved.
There is not, as yet, one central organisation with whom practitioners can register, so it is worth exploring and also seeking recommendation via friends or family.