Okay, step away from your feelings for her. Dude, sorry, but you honestly need to.
You don't have to fuck the office slut, but don't dwell on not doing so, for your former lover's sake. She doesn't care. She hasn't cared for a long time. She's paying you lip service, even if she doesn't feel she is, she is. You need to leave her be, move on, let her come back, if she is going to.
I've told you that you need to just not answer the phone if it's her. You need to let her know that you care for her well being, but that doesn't mean you're going to be the caring man she reaches for. That if she's got issues, she needs to work them out with the people she is with, and not those she just wants to be there for her and her only.
You need to cut it off at this point, other than for emergencies. And if she abuses that, then just cut it all off. Honestly, you're still obsessed with what she does and you do not. You don't need to start dating, but you do need to stop dating her or being "loyal" to her in your mind, regardless of whether or not you do anything.
Honestly, you're hurting yourself. She hurt you, yes, but now you're just hurting yourself far worse. It's over. She made her decision. She doesn't want to live with it, but it's time that you made her. Either that, or reside yourself to have a 3-way (or more) relationship that she wants. If you do not, which you have basically and repeatedly stated you do not, you need to drop her from all, new feelings.
Doesn't mean you can't love her. It just means you have to stop caring what she does or thinks. Honestly.