She was not an absolute monarch. There was a Parliamentary system in place by the middle of the sixteenth century, and limitations to the power of the English monarch stretch as far back as Magna Carta. She presided over a state that was virtually paralyzed by internecine warfare, as it had been in the Wars of the Roses. Remember, she had to fight her way to the throne past a candidate rigged into place by a noble that imposed his choice upon the dying Edward VI. There were serious rebellions during her reign, most notoriously the Wyatt. And throughout her rule she faced the vibrant figure of a young Protestant heir in Elizabeth, and Mary was never able to bend her to her will, or force her to convert to Catholicism.
She was not dour. Her history is filled with anecdotes of her love of parties, gambling, fine jewels and beautiful clothes. If anyone was "dour", it was Elizabeth, who carefully presented herself as a figure of sobriety. Jane Grey once returned a gift from Mary of fine cloth, designed to make a dress, with the remark that she would follow My Lady Elizabeth's example and hold to God, Who apparently liked plain fabrics.
If she was depressed by the end of her life, she had reason to be. Her attempt to reunite England to the See of Rome had failed, her attempt to have a child had failed, and her marriage had failed. Mary died in 1558 knowing that everything she considered important would be undone by the succeeding reign. Far from being an absolute monarch, she was an ineffective one.
I understand that people feel she held too much regard for religion, but honestly --- so did everyone else. Except Elizabeth, I think, and she turned out to be the most successful Tudor monarch, so there it is.