It has taken me 10 minutes to get on Bailey before and it's just embarrassing! Any advice is appreciated.
Dont be embarassed. Most students at our RS are "mounted" by the RI at the start of the lesson - me too since my op, but in the old days I did mount myself. I did just as you describe, used Mark Rashid's method and led the horse round in a significantly large circle till she stood at the block exactly where I want. I know other people (staff coming to the end of the staff lesson) thought me wierd and it "wasted" part of my own lesson time. But it gave me satisfaction.
Once I had taught the horse before two consecutive lessons - she remembered, until I next had a break from lessons - so it was an investment worth making. I dont use treats.
But the question of what you are doing to signal to the two horses that you want them to move their bums away from you is also important and would be easier to answer, if we had a video , or description.
It might show not just what you are doing but at what point the wrong message is reaching the horse.
I have done all the following - so this is not criticism of you. For instance, it is very easy as one prepares to mount to take up the reins and ever so slightly guide the horses head towards one. Moving the front end of the horse towards one, involves the back end moving farther away.
The same shortening up of the reins might be happening as you step up onto the block. I would usually prepare to mount with the reins already back over the neck of the horse. That means holding the near side rein which - if there is any pressure there - has the same effect.
My own problem was usually the opposite - asking the mare to take one last step forward with the near side front leg, to be where I wanted her, I would inadvertenly push the front end away from me. My RI taught me to overcome this -by reaching across the horse with my schooling whip and touching her off side shoulder instead.
Ideally one should lead the horse up to the mounting black in pretty much a straight line - so when I say circle round and come back to the block, thqat doesnt mean arriving at the block on a bend - I always approach straight. If your circle ends with a bent horse at the block, that might involved the hind end being farther from you and the horse moving it yet further, to straighten itself and balance its feet to accept your weight as you mount.
I ignored other people. I would take as long as it took. I also made no distinction between the failed attempts that were due to me and those that were due to the horse - This again is Mark Rashid's philosophy. The horse is doing what it thinks is wanted, or ignoring signals as to what is wanted - but it is not the horse's fault. It is not your fault either.
And that is why he doesnt use rewards. Because he doesnt use punishments either. The horse reads the signals - and as you say, when we get the signals right - and then repeat them till they are understood as being compulsory - the horse gets the message.
I hope this may be some use. I have taught RS horses to do as I want with no rewards. Because that is how I was taught - and for me it has always worked. But it is someohow inside our brain, what we find convincing and efficient. You need to make your own choice about that.