"...I don't know what the last 2 sentences of your post mean. Could you explain?..."

From what you've written, your relationship - according to your husband - was so toxic, he developed depression and he reacted by hunting for someone else, which introduced an external commitment on his part. During the juggle, you were discarded, with the explanation his treatment "opened his eyes" and revealed your relationship was the cause of his depression. 

So, unless you like being treated like an old pair of socks, your only two options are to either continue with carrying the spare baggage of unhealthy emotions, or finding the determination to discard the baggage and be the good person you are. 

Another thing: Do I think his depression will return? Sure it will. If he couldn't correctly handle a relationship in the past, he'll react the same again, and the new person in his life will experience what you experienced.