Ask yourself what need is it the new girl would be fulfilling and if there is a way for your girl to compensate it so that you don't feel like you need to fukk the new girl. if so, talk to your girl about it and direct her into doing what you need her to do. If not, go to your girl and tell her straight up that you want to stay with her but there is this other girl you're really feeling and you find yourself lusting over and that you didn't want to lie. Tell her she's free to leave if she's unhappy with this but that you hope she stays. Now of course you should expect her to be mad, sad and angry and be ready for that. If you're not ready for that, don't bring it up and don't sleep with the new girl (that goes without saying). But stick to your words and keep it real. maybe you could find a compromise (threesome, open relationship). If she wants to leave, don't hold her back. If she stays, i guess everyone wins.
disclaimer: this can only be done if you,re willing to be honest with her and with yourself and ready for her to leave. You have to evacuate manage fear and bytchassness within. whatever you decide, i hope it will be wise and provide happiness to everyone involved.