Oh, Sykes, you sound so sad, I wish I could give you a big hug. It sounds to me as if you are grieving so hard yourself over this relationship that you are struggling to cope with everything else. It isn't your fault, you can't help how you feel. It sounds as if you are doing all the right things for your dds, this is a crisis for them and you can't change that, but the support you are giving will ensure that they heal. Most children go through tough times of one kind or another, it doesn't mean they are scarred for life. It's whether they receive care and attention or not that will make the difference to how they react to it.
I know how sad you feel, looking at such little girls and wondering why they have to go through this. But they will be fine. Keep reassuring them that both parents are going to be around for them.
My dds were 4 and 1 when my ex dumped me quite unceremoniously. I lived in his house, worked for him and had lent him many thousands of pounds which I had borrowed on credit cards. So I and my girls lost everything. We were not married and so I had no claim on his house. My only form of recourse would have been a court battle for which I had no heart at the time. So I brought up my girls alone, worked hard and paid all the money back myself. Yes, it did scar my girls, because I was depressed and doing two jobs and I don't think I gave them everything they needed emotionally. You don't have to make the mistakes I did. You can make sure you get enough financially for one thing, it is a pressure you don't need at the moment. (You do not mention this so probably you have it sorted already).
My ex was a bully and I was afraid to stand up to him. But ten years later I was forced into a court battle anyway, which I won. I think if I could do it all again I would grit my teeth and have all the battles at the beginning so that I could then settle my girls down to better stability than I in fact did.
I was lucky to meet my DH five years later and now have a great marriage and a wonderful father for the girls. But the eldest is still suffering emotionally and now she is a teenager it is much harder to talk to her.
You are being very gracious in swallowing your own feelings to let your dds have their relationship with their dad. Well done. Don't let your hurt feelings build up, have a good rant on mumsnet. Accept all the help people want to give you. Take great care of yourself so that you can keep it together for your girls. And continue the wonderful work you are doing to patch this up for them. This may the hardest time of your life - we women are strong people. It is incredible what we can do when we are determined to give our children the best.
Are you anywhere near me in West Herts? I'd be glad to meet and help in any way I could, cos I really identify with what's happening to you.