This has happened to me...too frequently. I'll share one.

I was best friends with this girl in high school. After high school she quickly became engaged to this guy and my gut told me this wouldn't last. After a while I began to notice something... I liked him and the girl was a terrible match for him (and I'm not saying this out of my own jealousy, they really were a horrible match). So after three years of being friends together, which included spending the night at their house in the shadiest neighborhood, the shadiest room mates, and the mice and pests that overrun the apartment that I shared the night with, not to mention their using me for my car to take them grocery and craigslist shopping, dragging me into their constant fights, having me play the mediator when they didn't want to speak to each other, etc. But more and more, I thought he would be happier with me, and I really began to dislike my best friend because she was a basket case who thrived on drama. She was not a good friend to me, and I thought this guy and I really had something. Oh, and being a blind young girl, it didn't send any warning signs when he had lied about being arrested so he can drink with his friends while I and his gf went to three different police stations looking for him, instead, I thought we'd be a great match, and he only acted the way he did because my best friend was a basket case, that he wouldn't yell or scream or punch holes in the wall if he ever got mad at me. So this guy and I met in secret at his work after she had gone to bed. I thought we had great chemistry, and we confessed that we were both in love with each other. Then, I met him at a bar. He was drunk, and we made out. We drove to my house to have sex but my mom was home and I was not about to get busted having sex with my best friend's fiance. So we agreed we would tell my best friend that he was breaking up with her and we were dating now. And we did. It devastated her. She asked this guy if he could stay the night with her so she wouldn't kill herself, and he did. I couldn't sleep that night, I had felt so guilty for backstabbing my best friend. I was a mess at work the next day, and when I came home, I found a post of this guy who said he was never drinking again and that I was just a dumb, drunk mistake.

So yeah, I've had some bad experiences with being young and naive when it came to relationships.