This is probably super-obvious and I'm sure you've tried it already, but for me, I can only make the connection to the PTO when I am standing on the left side of the machine, holding the drive shaft with my right hand and operating the collar/button with my left hand. If the splines aren't lining up, I shift the PTO control (which is on the left side of my tractor) to the mid PTO position, which allows the rear PTO shaft to spin freely. Then, I can rotate the PTO stub with my left hand while pushing the drive shaft on with the right.
I guess this is because I'm right-handed, maybe I have more strength in my right arm to hold up the drive shaft, or whatever. If it is extremely hard to do from one side, you might try experimenting with doing it from the other side.
Another thing that can be helpful is to have a bungee cord stretched across the A-frame of the implement under the drive shaft so it takes some of the weight of the driveshaft while you are aligning the splines.