First time anal with a girl did not go as expected [MF].

This is the first time I’ve written something like this, and I’m also not great at writing in general, so I apologize if this is hard to follow. Now a bit about myself and the girl in this story; My dick is above average, at 7 inches length & 6 inches girth, and that’s the primary reason this girl and I are FWB. I met her at an orgy last November and we’ve been having sex every so often since. She likes thicker dicks and enjoys it when her cervix gets bumped by a penis during sex. I don’t fully understand how it’s pleasurable but if it works for her it works for me. Now on with the story, sorry it’s a long one:

Yesterday around lunchtime I get a snapchat message from a girl that I’m FWB with and she says “I want that big dick”. Feeling sporty I respond with “where do you want it?” and she then sends me a picture of her asshole. I’ve never done anal with a girl before so she now has 100% of my attention. The rest of the work day was dreadfully slow and boring, cause all I could think about was that booty hole. I’d thoroughly eaten it out before, but now I’d get to fuck it.