I had this exact issue with a training horse, to the extent that the first time I tried to get on him, I ended up in the dirt next to the mouting block and he was at the other end of the farm.
I picked myself up, dusted off, walked out to retrieve him (and was zero % aggressive or “showing him who’s boss” about), swung by the tackroom for a bag of treats, and then returned to the mounting block.
I pulled him up to the mounting block, fed him a treat.
Fussed around with the stirrup and saddle, fed him a treat.
Got up on the block, fed him a treat.
Reached over and fed him a treat from the right side, banged on the saddle, patted him on the croup, fed him a treat.
Put my foot in the stirrup, fed him a treat.
Put weight on that stirrup, fed him a treat.
Stood in the stirrup, reached over to the right side, and fed him a treat.
When I seung a leg over, he scooted a few steps, i asked him to halt, once he halted I fed him a treat. (No this is not rewarding him for scooting. It is rewarding him for halting and teaching him to ‘look for the halt’.)
I then rode as usual, and when we were done I hopped on and off 5 more times before putting him away. It is easier to practice at the end of a ride than the beginning.
After this session the problem was solved, and all ot took was treats for when he offered the right answer.
You catch more flies w honey, etc.