Why is MY LIFE so HARD for ME because of PREGNANT WIFE?

i feel really upset today. I'm a stay home dad and i do MOST of the housework. I'm 34yrs old and know i'll become a great writer one day. When we married my wife told me she would work and i could stay home, and concentrate on my writing. She has a career in Hotel Managment and its good pay. I do not have formal qualifications and i've a sore back so it does not make sense for me to work outside the home. My mum helps me 3 hours per day. We have 5 children ranging from 6yrs down to 10 months and she has just announced shes pregnant again 4 MONTHS. She works in a local diner (for us to have extra cash) 4 evenings per week and only started the 2nd job 2 months ago. Its helping alot financially and i think she should keep it up until at least 6 weeks before the babys born. However, shes told me shes quitting in the next month. I mean she gets great tips as she makes an effort with the customers. I cannot take a job, as my back hurts. Also i've agreed to look after my two daughters from my previous relationship (they 4yrs and 2yrs), and they were born out of an affair ( i felt neglected by my wife) but now all is fixed up and i have regular contact with my little girls and my wifes cool with it. However, i would like to be able to buy more little gifts for my two daughters. Also their mom is real cool. Shes only 24 and has offered to help me out more around the house with ALL the kids and my wifes making a fuss. Why is life so hard?