Not really a disparager, BUT. did you park the mower up on wood so that you could spin Shaft without the blades hitting the dirt? If the stub shaft on the tractor wont spin then the shaft on the mower MUST be able to spin. Take a long screwdriver thru the cross if you need to to be able to turn the shaft. Also suspend the heavy shaft with a bungie cord to the A frame so that the weight is not constantly wearing on you as you fiddle with getting the splines lined up.
Like you did the splines must be clean, burr free and lubed and should have a bevel on then to facilitate alignment. I used to struggle with PTO hookup, but after taking some of my own advice and doing it several times, I don't struggle any more.
What you should do after getting it hooked up is take it off and spin the shaft with that screwdriver and hook it up again. Practice makes hard difficult grueling jobs easier, to the point that they no longer are hard, difficult grueling jobs.
The difference between a disparager and a complainer is the disparager has gotten good at an onerous difficult task, and the complainer has not.
I know, I know, once you get it hooked up the last thing you want to do is practice hooking up, but if you will actually do it several times, you WILL get better at doing it. This practicing works for many things, PTO shafts, and women too!