Contact the place where the dress was purchased. Ask if they would accept a return of the dress and any accessories. If the dress was altered, or if anything was on sale (or clearance) or was a special purchase, or if the shop has a "no returns" policy of any kind, you'll know that the option of getting the dress back is useless to you. But it's possible that they might take the dress back, although possibly not for the full price.
Then decide whether legal action is worth the time and cost to you. If it was a dress that was on sale, costing relatively little, it might not be worth going to court.
But if it was a dress/accessories/fitting/alteration that cost thousands, as wedding gowns easily can, it may well be worth pursuing in court.
Find out from your city or county what small claims court involves. You can find out online or go down to the courthouse and get information. Maybe what the dress costs is over the small claims limits, and at that point you can contact an attorney.
You probably don't have anything in writing from your friend in which she promises to pay you, but you can get copies of your credit card statements, and any checks she wrote to you for those first few payments. Make sure to have them.
Then write her a letter and send it certified mail, with a signature required from her. Inform her of the dress shop's decision (that they'll either take everything back for a refund, or that they won't take anything back, or whatever you find out from them), and demand payment for the remainder of the bill that she owes you or return of the dress in perfect condition. Give her 30 days to make things right. Tell her that your credit is being ruined now.
If possible, call the collection agencies and ask them if you will pay what is owed, would they refrain from notifying the credit reporting agencies? Sometimes they will. Check your credit reports on Experian and Trans Union (you may have to create an account but it's free) and see if any damage has already been done to your credit. If you can, make the payments to avoid more damage and then pursue your friend via the legal system.