How did this happen? What happened? What now?

I am so lost right now... can someone tell me what is going on? My gf (26 year old) of 6 years and I broke up once last July 2012 (she broke it off) my suspicion was she was pursing another guy or seeing what else was out there, she came back in October 2012. She came back to me with a tidal wave of love, she did everything for me and was very genuine when she came back into my life. 2 weeks ago she decided to end it... again. I work in private security/money transport, not the most prestige job but I do ok I am clean cut, in shape and get hit on all the time while in uniform but remain extremely loyal to her throughout the years. She worked in factory environments but slowly move her way into the office (I am proud of her), she became the receptionist and started meeting and getting the attention of all the guys that work in the office area. Up until recently she never took interest in anyone and remained loyal to me. Lately I started with a new company/location and had to re-learn and retest in everything all over again and also adjust to a day/night shift rotation (something i haven't done in years). I think this put a great strain on our relationship. We did not fight or anything or even much argue however I wasn't able to give her much attention. I was however giving her over half my paycheck to get her out of debt so we can start with our plans for a house/family. She told me she doesn't feel the same towards me anymore and is moving back in with her parents. She says wants to remain "very good" friends and wants to continue with all the plans we had this summer, would like to still watch movies with me, still go on road trips etc. I asked her when she started feeling this way... she said exactly 2 weeks ago (beginning of March). Another thing that has happened very recently... Her 2 close friends including her best friend has just started dating new people and are telling her all kinds of "sweet" things that guys are doing and saying to them. I straight up asked her "Are you interested in someone else?" she said "Yes" she said she didn't want to cheat mentally/emotionally (I know she wont rush into anything physical) I found out the guy has been buying her lunch and is a senior manager in the company. He is much older (late thirties maybe forties) much more established and mature, she is younger and much more immature both mentally and how she acts. Even though I have distanced myself I do worry. Is he just playing with her and looking to get laid? Did I do something wrong? Did she get influenced by all her friend's new boyfriends and wanted to feel those butterflies again? It saddens me that if (big IF) her new relationship doesn't work out and she thinks I was the one. I can't bring myself to trust her to not do this again down the road. Part of me cares for her so deeply and wants to remain friends, but part of me feels like such a doormat in doing so. I believe myself to be a strong person but it hurts so bad when the person you love tells you they do not love you anymore. I am just so lost and confused right now...

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