Op, are you in the UK?
If so, you can go to court and get an access order saying when you can see your children and how frequently. Your ex does not have the right to set rules like who you are with or whether your family see them, unless there is a safeguarding risk. Call the Gingerbread hotline for advice on how to do this.
You can also open a CMS claim that will set out the maintenance you should pay. It will be a % of your income. Go onto the the CMS web site and enter in your income and other details to the calculator to get an estimate. At least it will be a predictable amount each month and you can budget for it.
Good luck