Why isn't facebook letting me message this one person?

[†Musicâ€]

So basically fb is working fine when I message everyone else, just not for my cousin. You see, we had this big disagreement on facebook, and we were saying yesss and noooo. We were copying+pasting big paragraphs of yes and no's and now everytime I try to message her the entire page freezes up!!
I can message anyone else fine, but when she messages me from my inbox or the side panel on fb, everything freezes up. What do I do to fix this? I even tried deleting the messages and sending a brand new one. Even that didn't work. It's really frustrating because now when it's time for serious talk we can't even message each other without it messing up/freezing.