OP I haven't RTFT yet, part way through. But just wanted to say I completely understand your anger and resentment, and this is entirely normal. It does not make you a bad mother.
My ex-H left when my two were babies so I do understand. He has no contact at all and I have no family help. At first it felt like he'd ruined my life, but now, several years on, I see it entirely differently and am glad he isn't part of our lives.
Babies and toddlers are relentless and you're on mat leave so you haven't had any space to process your feelings. The end of any relationship is like grief and you will go through stages of disbelief then anger, resentment. But things will change. Your children will become more rewarding, you will get to watch them develop and grow up, share wonderful times, jokes, holidays, and he will be nothing in their lives. I know it doesn't feel like it now but in the end it is him who is going to miss out and have no relationship with them.
Love for children often grows gradually, it is a bit of a myth that everyone instantly feels overwhelming love. And obviously, just to cope, you've had to emotionally disconnect from life to some extent at the moment. Anti-depressants can make it easier to cope with traumatic periods in life but they often kill all positive emotions stone dead as well as numbing the painful ones, so that may well be contributing to how you feel (even though necessary at the moment). On their own they won't fix it though. You need proper emotional support, talking therapy. As a recent mother you are meant to be prioritised for this (even aside from what's happened to you!), so has your GP referred you for this?
I'd also echo what others have said about support networks. You're lucky you have family to support, lean on them. I'd never have survived without my friends. You may well make some great friends you can rely on once your children start nursery/ school, and that can make a huge difference.
Being a single mum is really hard. There's nowhere near enough recognition of it, and we're penalised financially on top. It's completely rational to feel angry at being put in this situation. Your feelings are totally valid, and you need time to sit with them, allow yourself to feel them and process them. And then you can move on and decide what to do now. There is no "should feel", you feel what you feel. But, we can only play the hand we are dealt. It isn't fair at all and your ex is a shit, but it's time now to start focusing on the future, and I really do believe that you will feel differently in time.