Breastfeeding & supplemental feeding moms

Is anyone feeling resentful about breastfeeding? 
I have a 19 month old daughter and now an 18 day old son. My son literally wants to be on the boob all of the time. I'm starting to feel resentful in regards to the feeding. My daughter is spending most of her day in a play pen because my son can't stop nursing for more than 45 minutes. I'm starting to think about supplementing some formula in just to get a break and be able to do anything with my daughter. 
Anyone have experience? Or suggestions? 
PS I'm not resentful at my son so don't go there, I'm not looking for anyone's judgements I have enough of my own. I also have no help from 7am until 7pm.