First,
. This sucks, and it's not easy.
I am having similar problems myself, with not a lot of luck in sorting it out.
I suggest you speak to a lawyer. There are two things you can do for custody.
You can get together and come up with a parenting agreement, which is where you both agree on arrangements. However, this is NOT a legally binding contract (it can be made into a parenting order though, which is), and relies on both parents doing as they have said they would do. In your case this sounds like it's going to be difficult. If either party is violating the parenting agreement, or you are having trouble coming to an agreement, you can go to counselling to try and work it out, or you could make an application for a judge to give you a parenting order.
A parenting order is made by a judge, and it's a messy and unpleasant situation to be in.
It is legally binding though, and breaking it is against the law.
Either way, if you get the courts involved they are likely to try and send you to counselling to come to an agreement.
I am in this situation; Chloe's dad just does not want to commit to particular days, and I'm finding it challenging at the moment, because he is just telling me when he wants me to bring her over. So it's really hard; I have to live each day without many plans, just in case.
I took my case to a lawyer at the beginning of the break up because he had made threats about taking C from me (even if he says he was just emotional and didn't mean it, it was a very stressful situation for me - he is not a reliable person, and the thought terrified me). I got an interim parenting order, he got served the papers, and was very angry about the things in it.
We later talked and he agreed he would never take C, and that the best place for her was with me. So now all we have to do is go to counselling and come up with an arrangement so that the courts are satisfied that we've resolved our issues.
HOWEVER, he is not wanting to go now, and insists we can fix our own problems. But he still can't commit to days - there is a lot more to think about than he thinks (oh, if you get a lawyer, ask if you can have a "Parenting through separation" pack - it has a lot of resources in it for making parenting plans - very useful!). So this is all doing my head in, and I'm slowly going
.
Sorry for the life story, but just wanted to let you know there's someone else having problems.
My advice would be to talk to a lawyer. It's very hard to sort these things out when one party won't co-operate.