I still don't find myself feeling happy that my lover has someone in her life that she loves, so in that sense, I don't *get* compersion either. But when I step back and look at it differently, it makes sense. I'm happy that she is happy, just like I would be happy if she was enjoying a tv show or an activity that didn't appeal to me - even though I don't have to be happy that she has another lover who makes her happy - I'm just happy that she has *something* in her life that makes her happy. Her happiness is the point, not how she achieves it.
For me, it is central to how I deal with my jealous feelings that I allow myself to not just stop feeling miserable, but to also translate some of what is happening into happiness for her.