If the engine RPMs are going way up when you give it gas but the car is not moving or not moving as fast as it should at those RPMs then the transmission is slipping inside. If the fluid is full there is not much you can do except overhaul it.
I would visit your local transmission shop and get their opinion on what to do next. If this vehicle has over 100,000 miles…it’s probably overhaul time. If not that many miles then maybe just a new reverse clutch kit. An additive might help a little bit, but won’t fix it. Visit your local auto parts and get a transmission friction additive, like Lucas and see if it helps, but it won’t for very long.
Don’t just run out and change the fluid either. That old fluid is dirty and gritty which is acting like liquid sandpaper giving the clutches friction to pull the vehicle along. If you remove the gritty friction fluid you will probably not move forward either. Have your local transmission shop drive it with you and get their opinion first before flushing or changing the old fluid.
Reverse gear is usually the first gear to show signs of excessive internal wear.