Try not to be so hard on yourself. It is actually pretty common for an individual to write off such debts in hopes of reconciliation or receiving some other benefit from the individual owing the money.
Since the current child support order was never terminated or modified then he still owes you for the child support that accrued after you signed off on the arrears (unless you also signed off on termination of new child support). This being the case, if he attempts to obtain retroactive child support from you then you can simply respond that any such child support you owe him should be set off against the money he owes you. Judges are reluctant to order retroactive child support anyway so you should be fine in that regard.
Having said that, if the minor child has started living with the other parent then that is usually a legitimate reason to modify the child support. Because of this and unless the child moves back with you, his motion to modify child support will likely succeed. However, keep in mind that I do not know ALL of the details of your situation and to be safe you should try to speak with a local family law
attorney during a free initial consultation to discuss the possibility of defeating such a motion. Generally in situations like yours, the child support order is modified. Keep in mind that such a modification will cause a recalculation of the child support and will take into account any fluctuations of the parents' incomes.
You can find such an Attorney near you by using the following link to select your area in California and it will display contact information for such Attorneys:
I hope this has helped your understanding of the situation. Please remember to rate my answer positively if I have been helpful. Please let me know if you need me to further clarify my answer.
Thank you and good luck!