How can a mistress get her lover to leave his wife?

Find out what she gives him that you don't, give that (or something like it). The only way to end the affair (if the two people involved in it won't) is to fully and brutally expose it---to family, friends, co-workers (if these two work together) and any spouse/partner that the other woman has. An affair dragged out into the open loses the excitement of its illicitness and secrecy. I'd advise privately going to a lawyer and discussing your financial situation first, though. Be fully prepared for whatever occurs. If you've confronted your husband with the affair and he refuses to end it, it's only a matter of time before you decide that you deserve better and divorce him.