hey, not sure if it'll help. but i've been on the other side before. Having to leave someone even though the relationship was good.
i left an ex a while back, the relationship was good in my opinion even though my friends thought otherwise. what you don't wanna be thinking is that she was prettier than you. If a guy likes you purely on that fact then he's just shallow and not worth bothering with anyways. If he did leave you for another girl because she's apparently prettier, then he's not worth having anyways. Trust me on that one! beauty is in the eye of the beholder and all that... just don't let it get you down, someone will come along who treats you the way you want to be treated, who will do anything for you rather than you doing anything for them!
hope it helps