i am getting worn down

Dh and I have been fighting like crazy lately, and between that and the kids needing me every second of the day I am starting to get really worn out emotionally. Why arnt these kids the most important thing in the world to him? Why do I have to beg him to accept the responsibility of being a dad? He was supposed to help me out with them today, but instead he went out last night until 6am this morning, and was too tired. AAAHHH like im not tired? like i wouldnt enjoy a day off, and just wake up and say "i dont feel like being a mom today i'll just take the day off"....he thinks this is a part time job that he can dictate when hes a dad and when hes not. Its frustrating. And he hasnt seen the kids all week, i cant imagine going that long and then NOT wanting to see them when the opportunity is there. Maybe thats the problem, I live for my kids, he lives for himself. I just dont know what to do to make him realize his kids need him. I am so tired, and I really want to scream.