I feel for ya, dude. The majority of these folks are calling you immature and silly, but honestly, these feelings are tough to deal with.
I went through a similar thing when I was around 20. My best friend from high school was seriously involved with a girl, and started hanging out with a good friend of mine from gradeschool. They lived in the same town, I lived about 200 miles away. They hit it off, and I was happy 2 of my friends found another friend.
I started getting the feeling they were dating and confronted them about it. They both denied it and told me I was silly for thinking that. I believed them. It all came to a head at a party when the girl was talking to a group of my friends and one of them said something about the two dating. I happened to walk by at that second, and they looked at me like I was death or something. She ran out. At that point, one of my acquaintances told me that they were dating, and it was well known, and everyone that knew of it was told to not tell me.
I was also friends with my buddy’s girlfriend, and he was cheating. He didn’t want her finding out, I’m guessing. I would have given him an earfull when I heard he was cheating on a friend of mine with another friend of mine, but I would have been an adult about it. I couldn’t handle the fact that I had hung out with these people, together and separately, for months and months while they kept this huge secret from me. A huge part of their lives… my buddy was dealing with being a cheater and my other good friend was dealing with being “the other woman”, and they couldn’t share these feelings with me? Betrayed is a perfectly acceptable word… I was one of their best friends. They owed it to me to not keep these secrets.
I didn’t blow a gasket or anything. I just stopped talking to both of them while I dealt with it. They kept trying to get me to talk about it, and I kept refusing. It was obvious to me by that point that I didn’t have to be open with them about my feelings when I couldn’t expect the same in return. A little over a year went by before I was okay with the situation.
And honestly, I was perfectly okay with it. Eventually. It took a while. But the great part is that now I can talk to both of them like it never happened.
It’s been 5 years since it all happened, and I never give it a second thought. We’re all still friends, and one could even say we’re stronger friends then we were before. My buddy married the girl he cheated on. And as a very strange ending, I’m now dating the girl who was “the other woman”. My buddy wasn’t pleased when I told him, but I was honest about it as soon as I could be.
Anyway, I know not everyone deals with things the way I do, but isolation from the issue is what helps me. It may be worth a shot for ya… just don’t talk to either of them for a couple months. Let them do their own thing away from you until you’re ready to handle it. And realize that things do work out the way they’re supposed to in the end. Good luck, man.