If I was divorce in one state but now live in another but cannot afford to pay in full, in which state would I petition to get the amount reduced?

You would file your documents in the state court in which your divorce case was filed.  Changed circumstances such as a change in income would be grounds for a modification of child support, spousal support, etc.  File an Order to Show Cause (court form) requesting the modification of child support, spousal support, etc.  With the Order to Show Cause, file your supporting declaration signed under penalty of perjury.  Your supporting declaration states the facts supporting your request for a modification of child support, spousal support, etc.  Your current pay stubs will provide evidence of your reduced income supporting your request.  When you file an Order to Show Cause, the court schedules a hearing on these issues.  Check with the court clerk to see if you can appear by telephone at the hearing instead of in person since you now live in another state.  You may need to timely file a request for a telephone appearance and the court clerk can tell you whether or not you can request a telephone appearance and if so, the amount of time prior to the hearing that your request has to be filed.  If you don't timely file your request for a telephone appearance, it will be denied.

When you file your documents with the court, attach a proof of service and serve a copy by mail on your ex.  You can either use a court form proof of service or you can write your own.  The proof of service verifies the date of mailing.  If you write your own proof of service, it just says that you are at least eighteen years of age and the attached documents were sent via first class mail unless stated otherwise to _________ (name and address of your ex) on _________ (date).  You sign and date at the bottom.  The date you sign should be the same as the date of mailing.