A couple of months ago, I had an experience which spawned an ethical dilemma which I find fascinating. I had been in a healthy relationship with a girl for some time, but after meeting and getting to know someone else—a girl in my class whom I got to know in a perfectly platonic fashion, so I can't see any wrongs committed on my part at that stage—, I fell in love with this other girl, whilst my feelings for my girlfriend withered and died. Understandably, our relationship could not go on after that, and so we broke up. I think we are both better off now than we were.
However, assuming that I had an actual choice between (a) 'giving in' to my infatuation and breaking up with my old girlfriend so as to be happy with the other girl (it seems that we're also assuming no independent will on the part of the 'other' girl!) or (b) resisting my developing feelings for this other person to preserve the relationship I was already in, also assuming that I would, in fact, be as happy as I initially was with my old girlfriend, would you say that I did have a moral obligation to opt for (b), given the probability that our breaking up would render my ex unhappy, at least for a limited period of time? Do I have an obligation to seek out the most happiness for myself in all my endeavours, or should I prioritise the happiness of others?