Guess what? Chick doesnt want commitment to you, she's not fucking you after not seeing her, and she's acting standoffish. She fucked someone else. I'll say that again. You have a chick who is looking for reasons to NOT be committed to you, yes, at least one other dick has been in her during that time.

You're already attached, FWB won't work because the attachment itch will urge you to push for a relationship and she'll withdraw again.

A clean break. Why? 3 things: 1) you save whatever dignity and self-respect you have left, 2) you reinvest that energy into yourself to become a better person, 3) if she's ever going to come back to you it will be because you didn't throw yourself under the bus for another shot at a relationship with her.

Win win. Go through a some hurt now to get over the attachment, or a lot of prolonged hurt later by being her FWB and having most of your intimacy not met by having a purely sexual relationship with this woman you were at one point partners with.