The biggest problem that I see happened two weeks before Market-Garden started when Monty stopped to get ready for his big push. That just gave the Germans a badly needed two weeks to plug the hole in their lines.
I think that there was a bit of a competition between Montgomery and his US counterparts, and that might have driven Monty to make some bad decisions. Haste makes waste, and the “bridge too far” was a matter of pride taking over on Monty’s part. His desire to beat those “yanks” caused him to take unnecessary chances.