Actually, does your horse know what “whoa” means? I got a youngster a few months ago who was nicely W/T/C, but she would not stand by the mounting block. I looked up a lot of videos and then thought carefully. They say trailer loading problems are really a leading problem, so I put this into play with the mounting block. Horse wasn’t afraid of it, just didn’t want to stand still, so I spent the next 2 days saying “whoa” whenever I needed the mare to stop.
Take her out of her stall to the cross ties to groom, I’d say WHOA before clipping on the ties. Walked her into turnout paddock and said WHOA before turning her loose. On and on…with pats and occasionally a tiny treat.
Led her to the mounting block, said WHOA. As I turned the stirrup to my foot she slowly walked away, but I had hold of the reins. I asked her to keep walking and she did 3-4 circles around me (on the steps) and when she looked like she felt silly I said WHOA again. She stopped and I pet her. Tried to mount again and she was still. She was good for several seconds and I petted/praised like crazy. Took 3 days to be very trustworthy about it! She’s been good ever since.