2000 Plymoth Voyager 2.4 auto 4cyl

62,000 miles Transmission problems. Started happening when I drained as much fluid as I can from unscrewing the pan then filled 3 quarts of Mobil ATf +3 fluid about a month later approximately 600 miles it felt like it started slipping at about 40mph and especially up hill. Got worse very quickly started doing it off the light and in reverse/ Ii took it to a specialist they listened to what I did and said they added some friction modifier $79 that seemed to make the problem better for about 100 miles, but then the problem came back, they suggested a transmission flush and service $259 done at the Dealer I barely made it our of the parking lot. The car can still get around it shifts and shudders but keeps going. Am I out of options? Just drive it till it can no longer go?