i need help dealing with severly depressed husband of 3yrs

Hi. We have been married for 7 years but after my husband came back from his deployement 3yrs ago he has been diagnosed with severe depression. He is getting therapy but I see no signs of improvement. I need help in understanding his depression. We have a five year old, a three year old, and a 2month old baby. I work 13hours a day and am the primary parent. I am overwhelmed. I sometimes feel like I have been a single mother for 4yrs and I'm exhausted emotionally and physically. I've already told him this but due to his depression he is like a wall. He makes no comments. I want to be supportive but I'm just too tired to keep allowing him to sleep all day and isolate himself from his family. I love him but feel like I'm starting to resent him for his actions due to his depression. I need advice on how to cope with his actions. I get easily angered with him and find myself always counting to avoid an angry outburst. Today for example he left me alone at walmart woth a crying 2 month old and a 3yr old without saying a word to me. I waited by the exit for several minutes then had to find him in the parking lot in the car. T I had no idea where he had parked the car. And when I found him he hadn't the slightest idea of why I was mad. Either I have unrealistic expectations or he has lost reasoning... or maybe a mix? I don't know... someone help me before it gets worse please.

Thank you Karen. I talked to one therapist but she basically told me that for me to see any improvement she would need to see him. He doesn't want to go to any therapist except his and the lady I was seeing simply told its going to take time, to not look at the big picture so as not to get overwhelmed and that I'm doing a great job. (I'm not trying to be egotistical) So... yea not helping. Maybe try another therapist?