Check reins are a no-no with Donkeys- you can see she is not a happy Monkey by the cant of her ears!!
I would also strongly suggest you ditch this harness, basically it is cheap and nasty and not a correct...well, anything, basically!!
*[SIZE=8pt]sorry!!*[/SIZE]
If you want to persevere you need to get a loop and a tug stop fixed to the shafts- you can buy them on a lot of web sites and you can drill and bolt them, or, if you are handy with a welding torch you can weld them on, mine are welded, but they came on the carts...not sure why a cart would be made without them, but there you go.
You need to play around with the breast band, it is a bit high, but this is a difficult thing to get right on Donks, so it needs fiddling with, it also needs tons of padding...I hate these cheap strappy harnesses!!..Oops.... *[SIZE=8pt] sorry again, not your fault, I know...[/SIZE]* ditto the saddle, LOADS of padding needed.
Breaching should attach to the shafts further back, Saddle (tugs) should attach to the shafts in line with the saddle itself....oh, hang on, I can see a loop on the shaft...what the heck is that doing there??
What you really need is someone there in the yard with you, who knows about harnessing...any chance of that??
When you have padding under the saddle it will pull the tugs higher, so hold off adjusting them...take the tugs out of the loop on the shafts and put it straight down from the saddle..OK??
Heighten the shafts, and run the traces under the tugs, (not through them) and anything else in the way and straight to the swingle tree.
Tighten the traces until they take up pressure as soon as the animal starts to move forward, the animal MUST pull from the traces, NOT from the saddle.
Fiddle with the breast strap until the Moke looks happy.
Throw the check rein away!!!
Make sure the blinders are well clear of her eyes- a lot of Donkeys are driven in open bridles, over here very few are driven in blinders, might be worth asking her which she preferred?
A lot of Donkey Harnesses have a buckle at the top of the bridle so that you can open the bridle and get it on without pulling the ears through, my Mokes HATED that with a vengeance but when I made them bridles that opened between the ears they were OK.
That is a very nice Donkey you have there, and she looks ideal for driving...with a bit of time and effort you will get this sorted, so please do persevere.