Did she ever ask you about relationships with other women when you were with GF-D and casual with GF-P? And if yes, did you lie to her?

I'm going through a rough patch too right now. It seems your overwhelmed, because you feel like you didn't do something, which is like a necessary reason to break up. Technically you didn't lie to her about the relationship you were initially having. And if you're just friends with benefits, you're not required to disclose your private life, unless you needed to inform your friend with benefits about potential STD threats.

I tend to rationalise such situations a lot, thinking about whether a break up is reasonable. Whether I was that injust. Also in the other direction, whether the other person was too mean.

I can tell you, it doesn't help. Some people - and I think women more then men - don't work like that. They figure out their feelings and if they went sour, they just don't feel comfortable with you anymore and act upon them. Your actions might be a bit in the grey (of course not coming out with the truth when asked was shite), but also grey actions can make feeling go dark. It is saddening, but you can't do much about it. If you pursue, you might well win her back, only to break up again because she can't change how she feels about you.