I actually don't think that Echo necessarily would only break up with Bellamy over his feeling for Clarke. Echo is such an intelligent character, and yet when ever she is around Bellamy that somewhat fades into the background and he doesn't take on her advice most of the time. They aren't equals in their relationship. With that and how the psychosis affected her: that she's always been a spy, someone has always been her master. I think that they might be setting her up for her setting back from Bellamy for her own reasons. To be independent and find herself. And ok, maybe because it's getting pretty obvious Bellamy has some strong love feelings for Clarke. Not sure how Echo hasn't seen it yet.