I think nowcemsi that it's probably the 'it's not fair' factor that comes in. When my ex took off with a little tart I assumed it would be a fling and all blow over. At one stage they split up and although I knew he wouldn't come back I felt better because they weren't happy on the back of my misery. Now they are back together and all the progress I made seems to have been lost. I'm gradually letting go bit by bit I'm sure like you that if I see any signs of their being happy I will end up in tears. Frustration, anger, heartbreak I'm not sure which but it just doesn't seem right for them to be happy after all the heartbreak they caused. That's something I'll just have to learn to live with but the inability to be in control of my emotions really gets to me.
Not been much help I'm afraid but at least you know you're not alone.