How do i cope?

i am pregnant with my first child. Usually this would be a vary exciting time for me , but not only is it my boyfriends 3rd child we have custody of his first two girls one that just turned one and another that will be turning 4 soon. still not an issue right? well i am also trying to quit smnoking and since my boyfrien dand i cant afford daycare for the girls i spend 3 days a week home alone with the girls. The combination of the hormones and the lack of nicotine in conjuction with the stress of caring for two little children is seriously dragging me down. I feel like all i do is bitch at my man scream in to pillows and get frustrated with the girls. Its really not fair to the little ounes for me to be so irritated all the time but i just dont know what to do! I could really use some good advice if anybody has some!