What's wrong with me?

So I have this friend. We'll call him Fred. Fred and I became friends during the Fall semester last year. He was taking the class we had together with another friend of his, and the three of us studied together. He and I became closer, but we agreed not to "do anything" out of consideration for our friend, and others in the class.

During the winter break, I met someone, and we're dating. My feelings for Fred waned, but we're still friends.

Spring semester this year. We take another class together, and we're still really good friends. I can pretty much talk to him about anything. But someone I'm not really fond of (we'll call her Betty) was also in our class. Semester ended, and then Summer Session came around.

While Fred and I were talking, he mentioned that he has started seeing Betty. So I still want to hang out with him, but now she's there.

Well, yesterday, she pissed me off (if you make me think I'm wrong, then I find out I am, indeed, correct, I'll be pissed at you too) and now Fred is trying to make me feel bad for being insensitive. WTF????? Uh-huh. Don't go there.

So I walked out on him while we were hanging back in class (we're taking a class together right now, too), and not only am I pissed at Fred, at Betty, and at both of them for just being together, but I'm pissed at myself for being jealous. Why do I feel the need to feel jealous?

Well, jealousy is insecurity in a relationship. It doesn't have to be intimate. So now another person, who is helping Fred where I was helping him has come around, and I'm feeling jealous. I like Fred, and he's a really good friend. But I don't want him to start ignoring me because Betty and I can't get along. But I think I'm going to have to tell him, if he asks, that while Betty is there, I will not be.