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?
I'm sorry for your loss, and I doubt very much that you can keep a flight attendant on the ground, when her opportunities to meet and charm prestigious exciting and/or wealthy men are so everpresent. She may well get tired of her new BF pretty soon, but I doubt that she'll want to come back to you. She's more likely to keep moving on and trying to "trade upward in boyfriends" until she's ready to settle down IF she wants children of her own.
You can make yourself your new project: growing a stronger, more successful and satisfied YOU, and learning to enjoy your own company by making your own life interesting to you. You've probably not yet decided that you have to let go of trying to get her to come back; but once you do, you can start to rebuild yourself, and you can find a new girlfriend who makes you feel good about yourself, and who you can treat now how you wish you had treated the fly-away bird when you were younger.