Hi MollyBloomYes - going through this at the moment although I'm not due until end of May. My exP left me after our 20 weeks scan and was with someone else less than 3 weeks after! =/
I won't go into the whole background of it all now but I know a bit about how you are feeling.
As slow said, it's crap, it really is but the best thing you can do it distance yourself (where possible) from the situation.
It is easier said than done when kids are involved but I know I was driving myself mad with overthinking EVERYTHING! How could he leave my family for hers? What's she playing at getting with him when she knows I'm pregnant? Why has he chosen to raise someone elses kids and not his own? Why didn't he just tell me he didn't love me instead of leaving me pregnant? Etc etc.
I fully tortured myself until one day, I just sort of clicked that, like you say - he left.
That was his choice. Anything that happened afterwards with his new girlfriend was irrelevant as he had made that choice to walk away from his kids and you cannot control other people. Wasting time and energy thinking about him and what he was doing, wasn't helping me move on.
That is a decision he will have to live with now and take it to his grave and rather him than me - I kind of get comfort from that. That's not meant nastily, I just don't see how anyone could chose to be apart from children, let alone newborns but he obvs wasn't happy with us anymore. If he does regret it in future, it's his loss.
You're not pathetic at all, please don't think that. I somethings think about the GF and wonder what he sees in her that made him leave me pregnant but then again, when I find myself wandering down that thought path, I instantly change my train of thought so I don't dwell as it ruins my whole mood.
I think you've done so well to be where you are and try not to feel like you're going backwards - it isn't an easy situation to go through but I wanted to reach out to you and say it must get easier because I honestly thought I was dying at first, yet here I am haha.
Advice on how to deal with the ex, I'm not sure about because I haven't got that point yet unfortunately with baby not yet due - sadly he doesn't ask after her so I don't even know if he will be involved when she's born anymore.
They do take my toddler though (in a taxi - not sure how you would feel about that sort of thing) and they are already living together. I don't like this one bit but I don't argue it for the sake of her having that time with her Dad and as long as she is safe, which so far I have not been given reason to doubt then I bite my tongue and get on with my life.
I'm not sure there is much you could do about the ex at the moment because your breastfeeding and good for you but I appreciate it must be very awkward having him around and it's not something I am looking forward to.
Sorry this is long, inbox me if you would like to talk =) Big hugs 