I watched my ex husband insist on being in non monogamous relationships with people after we broke up, but working from more of a DADT perspective (not quite, but closer to what you have) and he ended up losing a partner who sounds like your Abby, who felt he wasn't really serious about her and left her for somebody she "felt could commit"
My advice is to ditch the DADT. Talk to your partners about the others in your life, about where they stand, how you feel about them, what role you'd like for them in your life (or hell, admitting if you have no idea where you see things going). If you think Abby and Betty wouldn't clash, think about having them meet or see if they are interested in emailing each other (you don't mention Cindy's role in your life so I'm kind of vague on that). Either Abby will be glad to know what is happening in your life, or she will make the decision that it's not what she wants and walk away - and as you think this might be what she's doing anyway, I don't see you have anything to lose.
As you say you're mucked up in emotions...I'm going to recommend you overnight (fine, 2 day ship) "Opening Up" by Tristan Taormino, and go through THAT with Abby (hell with every partner) and it will give you talking subjects, checklists, etc. If you are going off of some printed material it does two things - makes it seem more real that EVERYBODY deals with some weirdness and confusion when negotiating alternate relationships and that you're not just some horny fool who won't commit to one woman... and secondly if you get too nervous about a subject, it's right there in front of you and you can read off the teleprompter to get through some of the harder discussions.
Happiness will never come to those who fail to appreciate what they already have.