How do you fix feeling like a bad person?

I have a few friends, and it seems each one of them are pregnant. At least, my pregnant ones out number the non-pregnant ones. I'm so very happy for them. I really do wish them all the joy with the pregnancy. At the same time it breaks my heart. I see the progression and the joy of doctors appointments, ultrasounds, pictures, shopping, registries, picking out names, and the many other things they are doing and it's hard. I sometimes find myself in tears and I feel so awful about it. I am so very happy for them and I can't understand how I can be upset when I should be so happy for them.

I haven't figured out yet if it's the meds I'm on or if I'm just crazy yet, but my moods are all over the place. I can be incredibly depressed for a few hours and then jump up and be happy and playful. This stuff just really gets me down and I'm trying to work through it and hoping everyday for a miracle.