Feeling a bit down...

It's been about a year and a half since my ex and I broke up, 6 months since I had our baby and from time to time I really feel hurt from the situation as if it happened yesterday. 
I've been a lot better since, but I have to mask the pain, betrayal and feeling alone. I'm so happy with my daughter, such joy has been brought to my life after her birth... I don't like feeling so down because I feel a little ungrateful having her, but it has nothing to do with her I just have a huge void. 
Anyhow, today I heard he's involved with the woman he left me for and it hurt a bit. ( there was this whole story about him not being with her etc etc, but of course that wasn't true )
I believe every person of your past is a stepping stool to be with the one you're truly supposed to be with. 
I just don't like, and this is going to sound very selfish but..., the universe continues to bring them back together and I'm left so unhappy. I know they're meant to be, but I'm not in a happy place as far as a love life is concern, so I despise their happiness.
It's unfair someone can take so much from you and they be left with a happy ending. Anyhow, I just needed to vent. Right now I feel like breaking down when I know I shouldn't. 
Depression sucks.