Hypothetical:

A girl wants to be friends only, because she has come out of a previous relationship and is moving out of area in two months. But we still have "benefits" and she tells me it can end any time, I just have to say.
I go along with it for a month and a half, but find another girl I want to have a go at. I want to wait until I see girl 1 again to tell her in person. She asks if everything is okay when I dont answer some txt messages instantly (I was genuinely busy) and I say it's just 'cos I will miss her. It goes on and we end up arguing about how complicated shit got between us. And how she feels used.
I go and see her to talk it out (I am not a talkative person). She understands but doesnt know why I didnt tell her earlier, I tell her I wanted to tell her in person. She still feels used, I have no fucken idea what to do about that. We are still friends, she will be over for a week in 2 weeks time and I want to mention something to smooth it over, because I didnt use her.

What can I say?

Note to hypothetical self: Dont fuck your friends, no matter what.

Like I said, purely hypothetical.