Unjustified Child Support Arrears In California

My wife and I got divorced in NV, and shortly after she moved back to CA with our two year old son. After about a year of barely seeing my son I moved back to CA to be closer to him. During the move I quit both of my jobs and fell behind on one child support payment.

My ex took me to court last January, within weeks of the move, and child support was calculated based on my previous two months income, which I was no longer making but was still instructed by the court personnel that that was the correct way to fill out an income and expense declaration. Therefore, my child support was based on a $4,400/mo income but in reality I only made $2,500/mo last year after the move.

I did pay approximately 18% of my income last year in child support to my ex; and because I have made payments on my arrears she has received an excess of $1,230 over what the online child support calculator estimated I should have paid based on my actual income.

My ex has already told me she will not forgive my arrears.

Even though I paid as much as I could every month my child support is in arrears because I could not make the full payment ordered by the court in January because I was no longer making the $4,400/mo the amount was based on. How can I get my arrears reduced? Or is it a lost cause because child support cannot be retroactively ordered?