i would contact CSA and get your assessment changed to what is reasonable for your circumstances - you need to be able to live - child support is paid based on a very complex calculation, but the main thing is that it is within your means to pay

as to the custody, she cannot withold access to your child based on how much money you give her. your child is not a pawn that can be held to ransom (and i'm sorry she is doing this to you). contact the family relationships advice line - they can get you in touch with a parenting advisor or local relationships centre that can organise mediation and a parenting plan for access to your baby. if you fear she will not abide by the mediated plan, you can go through the courts to have it made a court order - if she doesn't stick to it, she's in contempt of court

payment you make (or don't make) is irrelevant to your access to your baby. even if you have the financial means to pay a large amount of child support and choose not to (child support will eventually catch up with you, so i'm not suggesting it for anyone!) your ex cannot withold access to your child