I d say...Get out there hit on a few new girls. Get yourself some NEW options. Don't hang around or present yourself as the 'back up' option, the ex will just take you for granted.

Don't listen to any of her bitching about the current Bf...reminder her that listening to her complain about shit stopped being part of the deal when the two of you split up.

The kicker with all this kinda shit is that usually as soon as you genuinely start moving forward with one of your new options that'll be the time the ex turns up at your house at 3am in nothing but a long coat and heels promising to do anything you want as long as you just take her back...

But then again complicated dilemmas are (imo) alot more fun than heart ache.