no sex

I am now 9 months post partum and my SO and I hardly have sex. The other day I found his old phone in the restroom and found out he was looking at naked girls and getting off on them. We literally sleep next to each other every day and I try to initiate sex but it never happens. It's so depressing, I would be a little more understanding if I wasn't willing to have sex with him and rejecting him but I'm not. Idk what to do..just feeling really down tonight for some reason and I needed to vent.