Am I too jealous, or do I have something to worry about?

I have been dating a girl for 5 months now, and I love her very very much. I treat her the best I possibly can and always try to be there for her. I know she loves me too and we do talk about the future. We both go to college and it is summer now, so we live in different parts of the country. We are still maintaining the relationship for we skype every day and even visit each other when we can. The thing is, she is the type of girl who doesnt like hanging out with girls and only has guy friends. I have always tried my best to cope with this for I understand that it is her nature and I dont want to control her at all. I am not jealous at all of her best guy friends because she has known them for a long time and I would just be a total freak if I got jealous there. However, ever since we have been away from each other this summer, she has made this new guy friend at work. I know she hates her job and she loves to hang out with this "friend" because it gives her an outlet to vent about it. She does tell me whenever they are hanging out and claims that they are "just friends". For a while, I never really cared because I trusted her and the fact that she loves me. However now she is hanging out with him more and more often, and when they hang out its always her going to his house alone late at night. She always tells me about these meetings, and calls me when she leaves, but I cant help but feel uncomfortable about it. Finally one day she stated to me that he "makes her happy" and that she wanted to hang out with him instead of skyping me. Every time they hang out, she never texts me either, which she does a solid amount during other times of the day. After this I tried to talk to her about it, and she started crying a lot, and got defensive on me. I wasnt yelling at her, but just telling her how I was uncomfortable with it. I am scared because I know this is how emotional cheating starts, and she does spend a lot of time with this other guy. Also she did not really tell me of most of her issues at work and instead confides it all in him. Now I am really torn and I dont know what to do. I dont want to be the jealous boyfriend and stop her from doing anything because I know that is bad and will hurt the relationship later. However, I cant get over some of these feelings and it really hurts me. I dont want to be in a relationship where I am unhappy most of the time because of this. At the same time, I love her too much and dont want to break up with her. Am I just being too irrationally jealous? Or am I at least somewhat warranted? The problem with her is that she has a history of emotional issues, so I know she loves attention from other guys, even if they are just friends, and this makes me uncomfortable. Also when we talk on skype or the phone she doesnt always seem to be "there", whereas when shes hanging out with guys like this I know they have her undivided attention. What do I do? I trust she hasnt done anything yet, but im afraid she might start liking someone else. Shes also 18 and fairly immature in my opinion, which also scares me when it comes to situations like this. After our fight, she stated that she would lay off hanging out with him for a while. I told her that was not what I really wanted and that I want her to do what she wants, but she continued to insist. I told her that I want her to keep hanging out with him if thats what she wants, but obviously inside I dont. I feel that she is mad at me now though because of my jealousy, and doesnt really see my point of view. I did ask her how she would feel if I spent all my time with another girl and told her she "made me happy", and she agreed that she wouldnt like that, but nevertheless I dont think she really understands. I feel that she is deeply hurt by this and cant shake the feeling that she will continue to see him, yet lie about it to me, which is in my opinion a lot worse. How do I get through this situation?