I know that to sit is to press “Insert” button, but when I press “Insert” my character isn’t at all no matter how I try.
I have to stand still to wait until my HP and SP refill which is very very slow. Cry*
By the way, I play this game with mouse and keyboard my joy-pad just broke a while ago so I can’t press “R1” or “R2” to sit of course!
Can anyone help me? 
Are you using the laptop? Then in order to sit you have to click fn + insert.
Try also checking the solution posted in this topic: Why i Cant SIt Down?
Using mac with windows and i sit with fn + enter
Thank you both of you for answer, but my character still can’t sit.
I have the same issue, I have changed the key binding to many keys, and not a single one make it sit.