Sasuke had the saddest childhood. He saw all of his relatives dead. And to top that, he effin saw his own parents get murdered by his brother. Then he eventually lived through his childhood wanting nothing but revenge. Even though it was eventually revealed that his brother did it for a good cause, it was still too much for a child to bear.
What Gaara felt was mostly emotional pain: being unwanted, viewed as a monster by everyone and being led to believe that his guardian and his father wanted him dead. But what Sasuke felt was pure, solid childhood trauma.