Poor baby and poor you!
It will get better. Let me first send you my unwavering support for you. You are the center of the universe for them, so do your best to remain calm during the shreiking. It's hard, I know. Even if you have to step away, it's better than boiling over.
I doubt strong-will is your problem. In my opinion she's too young for that. I think she's probably having pain, and it sounds like you're doing what you can on that front. Thinking of her crys as notification that she's in pain will make you more sympathetic to her, that's good. Doesn't mean you'll be able to always tolerate it with a smile, but at least it'll probably keep you from getting mad at her. (You are allowed to be mad at the situation, just not her.)
She could have an allergy to breastmilk- it's rare but possible. You might talk with your doc about that if the prevacid doesn't seem to help. And you might do a little research on it to see if your (her) symptoms match up.
Otherwise hang in there! It'll get better, I promise. Remember her crys are not a personal affront to you, rather a call for attention and help. Do your best, that's all you can do. Love her up when she's calm, and when she's not, well love her more then. Do your best to get some rest, frayed nerves are exacerbated by sleep-deprivation (I'm sure you know well). Know that you're not alone, and that we are never given more than we can handle, we just have to dig really deep sometimes.
I feel for you! Best of Luck! You can get through another day, and another, and another, and then it will slowly get better. You can do it! You can do it!
Sending you wellness wishes
J.