You could always use the old, "you're leaving me anyway, so what difference does it make if you tell me or not? Just tell me the truth so at least I'll know." Also, sooner or later, you are going to hear rumors from somewhere else. Prolly, anyway, and it would behoove her to tell you the truth so that when you hear the rumors, you won't go balistic.
Personally, I like the first one best..."you're leaving me anyway...nothing is going to change your mind...so please, to help me be at peace...or to help me deal better, just tell me the truth."
Maybe it's just me and I certainly wouldn't do it without getting someone else's input on it. Puppy?
Melissa
"Standing knee deep in a river and dying of thirst."