A case of "Grass is Greener"? Just dont know

Hi, I just want people's opinion on my situation. I've been with my girlfriend for 4 years known her 5. Good relationship, no fighting, compatible,emotionally supportive, dont go out as much due to money, some religious difference. 2 weeks ago I give her a kiss goodnight to which she responded under her breath "like a brother." I ask her if she loves me and she doesnt respond, I ask her if she's seeing someone and I figure out its an old family friend she's been talking to online since july. We were just talking about marriage in June. Next morning I ask her if she's breaking up with me, doesnt answer. She gives off all these reasons that seem fixable in our relationship if she had just brought it up to me. I told her to remember that spark we had when we locked eyes with each other. She tells me she needed time. Didnt breakup with me there I only found out later from her friend. I never got a good reason as to why we broke up, she just left me with a lot of questions. I suspect though that her reason for breaking up with me is so she can talk to this guy guilt free not a reason she would admit to, hence the confusion. I suspect they were talking for about a month online before the breakup. She says that she lost my love for me over time but there was nothing to indicate that in her behavior, we were talking about marriage in June. Her affection probably drifted to this guy after talking to him in July.Prior to all this she felt lost about her life, was unsure where it was going, with depression. Is she just caught up in infactuation? Is this a case of "Grass is Greener Syndrome", if it is what can I expect in the future? This guy she likes lives in washington, she lives in san diego, they skype, he is a known cheater, millitary guy in the navy. What are the chances of their relationship succeeding? Is this a rebound relationship for her? What do I do? What are my next steps? Thanks

It isn't a defined rebound, it actually sounds like she left you for him. A rebound is a post-breakup recovery to stop thinking about the breakup itself.

"Losing the love" is a common breakup reason, the love is gone and she moved on. Even if her new relationship fails, I don't think you'll ever be with her again. Sorry for the bad news.

thanks for the perspective.

I think you can't do much at the moment except accepting the fact that she has lost her affection on you. This happened a lot in relationship when one party just don't feel like going on with another party anymore. It's not because they don't love each other, it's just the other party has started to drift away maybe she / he is in need of something new. They may want to have a new lifestyle, new breath of air, new way of life...new bf / gf....etc. Something you totally can't control. But there is one thing for sure, when this happen, it means both of you are not fated to be together. Sorry about that. However, if you really love her, you could give yourself a time frame to be her friend, stay with her, and see if the situation will change. Because there might be times she got lost, and she had made the wrong decision coz seduced by the other guy, there is the slightest possibility (it doesn't happen on every case, not that if you are lucky enough) she will still come back for you when she 'awaken' after getting hurt / cheat by the other person. At the moment, you still unsure if this is her temporarily back off, or she really has lost her affection on you. I guess time will tell. Good luck mate !