I have no clue when Michelangelo said this, but I do like the mentality of what he said. Michelangelo was a learner. Not only does this quote point in that direction, but take a closer look at another thing he said that totally tells us the same story, only with different words.
"The greater danger for most of us lies not in setting our aim too high and falling short; but in setting our aim too low, and achieving our mark."
Not only was Michelangelo learning, he was also stretching out for more because he wanted to achieve more. Someone who is learning constantly and yearning for learning, will discover new things that make them able to achieve more. And when you have a goal for learning, you will be able to see more and thus set goals that you earlier didn't even know were possible.
What are you learning? Are you stretching yourself for more? Are you simply living to survive? I know it can be hard, and life can feel depressing at times, but one of the best ways to counterattack such feelings is to stretch out for more and to set goals that might seem impossible today.
I hope you will have a great day!