Just take it day by day OP, just like you are doing, and you will get there. And don't beat yourself up: there's nothing wrong with you....but there appears to be a shit load that is wrong with him:
-You have mentioned he always put his mum and sister first - imagine having a relationship with someone where you are bottom of the pile of your OH's priorities, particularly with kids in the picture too. Looks like you dodged a bullet there.
-Look at how awfully he has treated you. 5yr relationship, living together, promising you the world....then throwing you out with absolutely no warning, no talking through of concerns/unhappiness etc. Either he's the world's best actor, or he's had his head turned. Either way, that isn't the sort of person that will make you feel safe and secure. I always thought that most normal people would voice their unhappiness and at least try to to work on figuring things out, rather than going straight to, "right, you're dumped, get out now". If you did get back together, you would forever be worrying about the next time he pulls a stunt like that. Even with the legal protections that marriage gives, you'd still worry. It's just not worth it.
Years ago, when I was in your position, a lovely friend of mine told me that things would be ok, that my baby was a gift and that I have got the way better deal than ex. My mate was right. My DS is fab, he's a lovely boy who is doing well. He had a fab upbringing, surrounded by people who love him ---> that is down to me, family and friends. He's 18 now and knows who has always been there for him (I've not had to say a word). My ex (DS's Dad) is still overseas so main contact is through FaceTime, phone calls, messaging and the occasional visit. Ex has missed out hugely, and I know that is something that he now regrets.
Also....definitely start having a nosey at cool baby stuff, and furniture and just brainstorm nice things like that. I found that really helped me at the time!!