Francis was sickly and after their marriage, he died after a year.
Thing is, Mary wasn't brought up to rule, she was brought up to be a consort, wife, ornament to a cultured court, not as an administrator.
During the critical years of her development, she was spoiled and pampered and feted, not at all required to learn and participate in the day to day running of the country, or her country.
After Francis died, her charmed life ended and she had no idea how to handle it. Her whole life was then dedicated trying to recreate the carefree effortless past of her childhood and teenage years.
Then she went to Scotland and couldn't handle it.
I don't blame her for not doing too well, her nobility was bought and paid for by Elizabeth and her nobility spent so much time asserting themselves against her that she couldn't accomplish anything at all of substance.