Firstly, please no bashing. I don't need to be made to feel like any more of an idiot, I need a cuddle and some support.
DP and I have been together for 2 years, and have DD (4 months). He struggled a lot with my pregnancy and becoming a dad, and actually kissed someone else when DD was 10 days old when he went out for the evening. It wasn't just a kiss, as he was Facebook messaging her afterwards about how he wished it had gone further with some pretty explicit things he 'wanted to do to her' and I found out. After a long discussion ( more me reducing him to a crying mess and kicking him out for the night), we looked into the possibility of him having a man version of postnatal depression and worked on our relationship to resolve the issues he had - I also vowed that if he ever did anything like this again, then I'd be gone with DD in a shot.
Since having DD, we have been intimate but not had sex because I'm waiting on a revision after my episiotomy as I was stitched too tight 
I know he's felt a little neglected as I'm so wrapped up in DD and I've tried to get the balance right but it's naturally a struggle because she really is my world. I've been co-sleeping but have started putting her back into her cot because I know he's not been keen on it, even though he'd never say anything in reference to my parenting choices.
I've recently started working Thursday nights to bring in a little extra money for us which means I stay at DM's house and she looks after DD while I'm at work. I've just got home from DM's and flicked on the TV to find adultsmiletv as the channel which was on last which I'm not annoyed about, if anything I find it a little funny. But being the slightly on edge new mum that I am, I looked at his Facebook and found a 'hello miss 
' message to a girl, and then looked at his browsing history and saw that his Yahoo account had been used a lot so I looked.
There's a ton of explicit images sent by a variety of girls downloaded from his Whatsapp, including ones from last night while I was away, and also videos but I can't see those as I'd have to download them.
What the fuck do I do now? I am petrified and feel sick to my stomach. DD is so tiny and I feel completely at a loss. Part of me thinks that I care so little about myself that maybe I should just forget about it because he pays the rent and lump it. The other half of me wants to pack my stuff and go, leaving him to work out what he's done for me to leave. He's back from work at 8pm and I don't know what to do. Fuck. I'm a mess.