Welcome to my couples workshop, where people 18-80 of diverse cultures & orientations have come to explore their questions and find a path of heart. The system wouldn't let me do chat with you, so I had to switch it to Q & A to start writing.
I'd say 7 months is pretty long for an extrarelationship affair to last. And the roots of the problem might be in your not moving further toward a permanent commitment with plans for greater things (like marriage & children)--because sitting on a plateau of love relationship for several years after the initial romantic passion normally simmers down at year 3 could have made your GF internally frustrated even if she didn't express or even realize it.
Do you have any ideas about what she might be experiencing with him that she couldn't experience with you, besides "new love" that regularly "chases away old love?" Perhaps Some types of togetherness, like intellectual companionship? play, sports or leisure activities? artistic or aesthetic appreciation? creative projects? outdoors adventures? more socializing? or more alone time? more prestige? more or less interest in having children? religious or political beliefs?