I know, it is very hard sometimes when they are pitching a fit and acting horrible; it makes YOU want to pitch a fit & act horrible. But of course that is not a good idea. I see you have a new baby too. I definitely noticed my older son had some behavioral problems after my younger son was born & I also noticed I had much less patience with him after my younger son was born. This is tough. I definitely think if you want to stop the cycle of anger, you need help. Do you have family around you? Maybe once a week your mother, MIL , your dh, or even hire a babysitter, can take the baby & you can spend focused one-on-one time with your oldest. This can really make a difference. Also, carving out time for yourself to exercise & enjoy something you like doing can go a long way towards giving you more patience with your kids. I know it's almost impossible with 2 children, but I go to bed early & get up early to get this important time to myself. Lastly, Dr Sears Discipline Book & a book called "Becoming the Parent You Want To Be" have been helpful to me. Good luck to you. I know you can do this!
Stephanie, mom to 4.5 yr old Neil & 9 month old Reid