that sounds like something with the tranny
the shifter shouldnt be hard to move at all but the clutch/brake should be on when you do it.
i have found some have a small plastic thing on the shift shaft that can wear and get out of place to make it hard to shift
its a very simple system, pull off the deck and get under there on the shifter side and you will see its just a shaft that slides in and out of the tranny to shift gears. disconnect the rod and move it by hand and see how it moves of if its hard to move. if its hard to move buy a parts mower with the same tranny and swap yours out. or just buy a mower that needs an engine and put your engine in it.
i have learned the hard way its never worth it to try and fix a bad tranny