Auto gear boxes have 2 clutchs in them usually. What has happened the clutch ( about 12 friction plates) have lost the friction material and you find foward works great but no reverse. usually the forward goes but you can still reverse.
Either rebuild or get swap.
You must use the right oil. about 3 yeras or so ago all manufactures changed the make up of auto fluid. It causes problems with some boxes.
Also old boxes with asbestos belts began to fail. Also some jap boxes especially misubishi/daewoo