Thanks for the additional information. It was not clear from your original description that you were aware that he is depressed or that he is considering leaving the family. I am sorry to hear that.
It can be very stressful when you cannot get someone to talk to you. And if your husband is depressed, keeping his thoughts and hurt inside can be part of why he won't share with you. Getting him to talk about his feelings can seem overwhelming.
What you can do is to keep encouraging him to talk. He may not seem to listen, but it may take several tries to get him to open up. And if you feel he will not listen to you, ask someone who he might listen to such as a close family member or friend. It is too hard to go this alone and support can help a lot.
Also, try talking to his doctor to see if he can help. Your husband may listen to him.
And if you feel your husband might hurt himself, then it is very important to talk to a professional about your husband's situation. His doctor or the local ER can help.
He may not be able to tell you why he feels as he does. Depression can build up over time or it can hit all at once. He may not know himself why he feels this way. And motivation is a huge factor with depression. So with that in mind, being there for him has more impact than you think. Just sitting with him or offering to help him may help. Here is a link to help:
Most of all, take care of yourself. It is very stressful to cope with someone who is depressed. Talk about how you feel, consider therapy for yourself, get as much support as you can. Spend some time away from home and enjoy yourself for a little while. It can give you time to organize your thoughts and to approach the situation with less stress.