When something happens with my friends, when we get into an argument or I did something wrong, it always keeps bothering me for days.

Like a couple of days ago, I told my friend D. What was bothering me. But if my friend are. Hadn't told me a certain thing, I couldn't have said something about it to my friend D. So D. Knew are. Told me something about it. I told are. That I told D. Because it really bothered me. So are. Got a bit annoyed that I couldn't keep it to myself. But I only said it so there wouldn't be any problems anymore.
Now are. Told me something that someone told her about me, and I mailed that person about what he said, because I felt the same way even before are. Told me. I didn't say are. Told me something.

Now she's like: you told him I told you? (which I didn't) and she got a bit mad at me and was like: "i won't tell you anything anymore ". But I never told him that are. Told me. I said to are. To tell him I thought the same thing and that's why I mailed him.
I said sorry, that I didn't know he would connect the dots between me telling him and him telling her. She said it was okay and we talked about other stuff.
But it still bothers me. It wasn't my intention to make him think that she told me. I didn't tell him she told me. Now I'm kind of scared she won't trust me again or tell my friends things I told her about them.

To generalize it, I'm upset with the most little things that happen between me and my friends. Like this is going to bother me for days, even though she said it was okay.
I don't want to be upset about every little "bad" thing that happens between me and my friends, but I do it.
Anybody have any ideas how I can let things go easier?

Sorry if it's a bit confusing...

You have several short, meaningless sentences at the beginning, even though you write quite clear ones at the end. What's with that?
Anyway, I get the message. If you tend to hold a grudge, look around to see where you might have learned this trait. A parent, a sibling? It is possible to change, but by the time you are grown (you don't say how old you are) it is harder. You made the first huge step by recognizing that it's your problem and not others.
Three ways to change: write it down in a journal, sleep on it, and talk about it with the person it's about. Oh, and a nice heartfelt apology does wonders. Don't go on and on, because that tends to get into reasons and reasons turn into excuses, so just say 'I'm sorry.'