1998 Mercury Villager

Originally my transmission was changing late, it wouldn't change gears until 30 miles per hr. Now, It will not switch gears at all. When I put it on drive, it stays on low gear while i'm driving. It will not go automatically to the next gear. Reverse works, Drive works, but it will not shift automatically or manually while i'm driving. If I shift it Manually to the first gear and then to the second gear and then into drive, it will stay on the first gear. My transmission fluid is good nd filter is good. What part in my transmission would be causing this problem?