over-reacting

i am a 23 year old mom and for some reason all i seem to do is over-react about everything. Its mainly with my boy-friend's kids. i hate the fact that i am so up-tight about everything. I try not to be but i get anxious if i don't do or say anything. when his kids are here on the weekends i get stressed out because they don't listen to me, they are very picky eaters, they fight for attention, and they whine about everything. mind you my boyfriend has m.d and is in a wheelchair. I have to take care of him and his kids and mine. its alot to do for a 23 year old! but i manage! i need advice on ways to just stop over-reacting about everything. hes annoyed by it and so am i. all i want to do is be happy and just to let things go.