It's complicated

Hi, I am currently in a relationship (engaged) with a girl, we have been together for almost three years and we recently moved into a rented flat together. I have also recently met an amazing woman at work. she is 13 years older than me (36) but to me she doesnt come accross as a mother or look older than myself. She is married and has two kids, but I already know that she is not in love with her husband, and would like to leave him, but feels she cannot because she doesnt deserve any better, and it wouldnt be fair on the kids. we both know each others situation, but we have been communicating secretly via usb sticks at work, and emails. I dont like being dishonest to my partner, but i dont get the same nervous feeling with my current partner than what i do just reading to what this other woman has written to me. we recently met up, and we got very touchy feeling, and ended up sharing a few passionate kisses. She feels she is now a sinner, because she owes it to her usband to stay with him at least until the kids are grown up (at which point she will be 50) I just don't know what to do. I feel like telling my partner that i dont feel the same. i want to tell this other woman i want to start a relationship with her. she normaly lives in newcastle, whereas she is currently in Oxford where i live while she studies. any advice?