I think I understand what you mean, but I never find the feeling of jealousy in those kinds of situations. I guess it's just sadness, a kind of sadness over what I know I can't have, sadness over that kind of happiness that I know I won't have again. Shame tends to come in as well when I remember how I got to this point, and how I'm responsible for throwing everything I had away.
I think "loss" encapsulates this whole feeling for me. Seeing others happy, content, enjoying themselves and one another. I used to have that, but over time I lost all those things that brought me happiness, so now I'm just left here. Alone, disparaged (to put it dramatically). And even though I want it back, I know I'm not owed it, nor do I deserve it. In the end, I'm glad that others find balance and can enjoy good times even when they come with bad times. I wouldn't want everyone to feel the way I do, after all. It's just depressing to know I don't have that propensity for balance anymore.