there could be number of reasons i will try to touch upon each
- low battery voltage damages the shifter .. most shifter have a lock in parking then a lock preventing to go in reverse while in forward gear .. may be that lock is broken out of place etc
- then there r solenoids to regulate oils pressure as per gear selection, may be solenoid is not working though i do not know if it effects ease of shifting
- if ur clutch gears r worn again shifting will be hard and jerky .. noisy or any of these
- oil level low or oil is old then again this could cause hard shifting
u need to go to gear expert shop and get gear box checked