Unlike some other posts on here I will not blame neither you or your girlfriend... that is if you guys are still together. What happened here is the classic case of two people getting into a serious relatonship at such a young age and expecting everything to be good without considering the fact that over time both of you will change in the way you think, feel, and at. I don't blame you for sleeping with this other girl because if you hadn't done it it was still going to happen later on. I've always believed that it is better to go through you trial and tribulations now at a young age, than be completely blindsided in your relationship when you are much older. I too dated my ex for 8 years at a young age and looking back now I am super glad that we are not together anymore. I've become a 100% a better person because of that experience but it didn't happen over night. I had to go on many dates and be put in many a situations to see the whole picture. For you to have had this one encounter and to think that you will not have those feelings anymore is completely bull. There may be at least 1% of truth in what you are saying but I think you are speaking from fear and the unexpected since ou've been so comfortable dating her for so long that you don' want to keep pushing forward to grow so that you can really know who you are and what you want.
Let me ask you, do you honestly think she has not been feeling the same way you felt before you dated your co-worker? Of course she has! I just think she had a little more self control but more so I think she had not reached the level of frustration that you had. My conclusion is that if you had not done what you did she was going to do it at one point or another too. Why do you think she hooked up with the guy soon after you broke up with her? Why do you think she took you back after you told her you slept with someone else? I think she did those things because she understands where you are coming from. She understands that you may have feelins that you may want to explore. You may be a beter communicator but that does not make your actions more noble. The real of thumb is this: If it happens while you guys are broken up then it is non of your business. Which explains why she didn't tell you a first. Just because you told her does not mean she has the right to tell you something that she feels is unnecessary to the survival of the relationship. Again, your actions is based on your thinking that she is "the one" and that she is this perfect work of art because she is all you know. Both of you made the ultimate blunder buy coming back to each other so soon. Give it time and experience the world without each other. You guys may never become boyfriend/girlfriend again but at least you guys can salvage some type of relationship. Honestly, you guys are/will destroy anything good in the relationship by remaining together at this time because you guys do not trust each other. Not only don't you guys trust each other, but one of you is bound to feel constraint again and do the exact samething. Its only a matter of time. Be smart, swallow your pride, and walk away from this for a few months and then see if things are still the same before you decide to jump right back into a relationship which you had given considerable amount of thought to leave from. Good luck.