Peaches, Hang in there. I have a 3yr old and a 17 month old. They scream at a frequency level so high I think my brain is going to explode. I have to remove myself from the setting so I dont come unhinged. Even then sometimes I do go nuts and the wife sends me to my room. Adult timeout I guess.
Your anger and that useless feeling are so common. All I can say to you is keep fighting this for your child. Find whatever sort of support system you can. Family, friends, whatever is available. I have found that my children have been changed by my PCS. The 3yr old knows that daddy has "mean times" and kills me to hear this.
As the previos poster stated, FORGIVE yourself. Find what time you can and enjoy something that makes you smile. We all are "good enough" for our kids but it starts with good enough for ourselves.