Have i gone from wife to mistress?

my husband left me for his young mistress almost a year ago,we have not yet divorced just seperated. We seem to be quite good friends and now are as my husband puts it"friends with benefits". I cant help but have mixed feelings about it. I know people may think i am crazy but i cant help but think that she ruined my relationship between me and him, why can i not do the same in return. sort of a "paybacks a b*tch kinda thing. He is still my husband after all. The thing is she is to gullible to believe the truth. Should i feel bad?