Re: Change in Custody Order
Custody and Support Laws vary by State. You didn't indicate what State you're in (or rather what State the Order is entered).
Any response to your query would be speculative without knowing the State.
So, given that, just speculating, yes, you could file a Motion to Modify Custody and Support. It's doubtful that you could ask for back child support, since there was no order in place during the time you had the child full-time.
The time for you to have addressed her failure to follow the Court Order was back when she failed to follow the Court Order. You can't unring a bell.
Re: Change in Custody Order
As a general rule you will be able to go back to court and ask that the court issue a new custody and support order that is consistent with the status quo - the manner in which you and your ex- are actually dividing custody. Sometimes the filing of the order will inspire the other parent to start insisting that the custody order be followed to the letter.
You will not be able to get the court to retroactively modify support. You should expect that modification will start no earlier than the date of your petition.
Given that we don't know your state or how your spouse might react to a motion to formalize your custody practices, it makes sense for you to consult a family lawyer in your state.