do you really love your wife? don't think so. she is just

your responsibility, you stay because pity her. you can either fool yourself or/and try to love your wife a little bit more, so you can live like many "happily" married responsible couples; or you can leave your wife, let she hates you all her life, and tell the other girl your feeling before she gets married. this way you might get what you want or loose everything you have. at least you are honest to everyone and will stop thinking about her.