Don't like that your LTR has an N count of 20? 30? Etc? Then leave.
Found out that your LTR used to sell her body for coke? Then leave.
Don't like that your LTR has been to jail? Then leave.
Don't like that your LTR had a train ran on her in high school? Then leave.
Don't even like that your LTR confided in you that she had a passing thought to sell herself for extra tuition money? Then leave.
You are not obligated to stay with someone whose past makes you uncomfortable. If you ever told your LTR why you were leaving however, she'll call you "insecure" and say that "the past is the past why can't we move on?" It's simple, we as humans are prone to recognizing patterns, this includes patterns of behaviors. You don't want to be with someone who you feel will be predisposed to reckless, impulsive behavior. It's hard for you to recognize your LTR as a support system if she has never truly supported any man in her life.
She may be right, she may have genuinely changed, she may have looked deep within herself and tried to reprogram herself just for you, but still the fact remains if it bothers you too much then do you both a favor and walk away. Let her be with someone who is completely comfortable with her past.
Remember that if you choose to walk away do NOT explain yourself. You owe her no explanation. If you must feel the need to say something, say that you aren't happy with her. That's it.