I wish i could help you too like leonie, but i'm too far away.
i had a similar life as you as my 15 year marriage ended when my three children were 3yrs, 8yrs, and 10yrs old. My children's father who i had been with since i was 15yrs old has only ever seen them once over the years which is really sad. No matter how much i begged, screamed, took him to court it made no difference to him. It didn't matter that i was sick either, all it did was dig my hole deeper and made it harder on my children.
Yes it is very frustrating and lonely being a single mum and very hard on you especially with BC. One bit of advice i can pass onto you is move on and dont let him ruin the rest of your life. It is very hard but you have your beautiful girls to help you. Your girls will survive this time if you move in the right direction.
After years of fighting with him to see his kids and going through major depression myself, i decided i had to move on and fight for myself. I went back to Tec/tafe full time and got some qualifications behind me, then got a job and moved up the ladder into lower management. I Built my own home and brought up my children the best way i could with the love and support of my own family. It will be hard but you are strong and beautiful and have survived this far through this tough journey.
I decided not to settle with another man in my life while my children were young and at home. Now that my children have grown up and left home I do regret that decision i made so many years ago..I wish i had looked after myself too. Be kind to yourself and build a future with your girls and you. It is his loss if he chooses not to see his girls.
All the best Cheers Jo