If your girlfriend won't go into counseling and make a change for the better, i would leave her, asap. Life is too short!!
Unless your girlfriend can learn to control he anger, i wouldn't stay in the relationship. Many time one will deflect responsibility for their own anger issues, by creating a situation where you feel you feel you have no other way to respond, in order to be heard, than to get angry yourself. In other words, her yelling all the time could be what is changing your way of communicating. Relationships are supposed to bring out the best in the other person, but that's not happening here.
Life is too short to be demoralized by someone constantly, it's abusive. It's not a good example for your daughter. If this is something that she's in denial about and you cannot hope that she changes it's a dead end relationship. Don't overcompensate for the fact that your girlfriend is sick, the fact that you are supporting her through this all the while being understanding that your sex life is taking a back seat. should warrant her gratitude and respect for you. It sounds like she is treating you with the same dynamic as her poor children. You are therefore enabling her, by allowing her to treat you this way, and it's going to end badly for you. Why waste your life on someone not able to be grateful for having you in theirs lives? There's more fish in the seas, my friend. Take a chance and find someone with less baggage, if she cannot be reasoned with. God bless you and your child.