What are the burdens that Holden carries? I know the death of his brother is one but need to know what else.

In addition to Allie's tragic death, Holden also witnesses the suicide of one his classmates while attending Elkton Hills. Holden elaborates on this traumatic experience in chapter 22 during a conversation with Phoebe. When Phoebe asks Holden to name one thing he likes, Holden can only think of James Castle. Holden recalls the time when James Castle was being bullied by his classmates and refused to take back something he said about Phil Stabile. According to Holden, Phil Stabile and six of his friends locked James in his room and proceeded to beat him severely.
Holden then remembers hearing the sound of James Castle's body hitting the ground. Holden also recalls running outside and witnessing James Castle's lifeless body mangled on the pavement. James Castle was even wearing Holden's turtleneck sweater that he let him borrow.
Holden also alludes to adults sexually molesting him or making advances towards him at the end of chapter 24. After Mr. Antolini wakes Holden in the middle of the night by rubbing his head and admiring him, Holden immediately leaves the apartment. Holden then says,

When something perverty like that happens, I start sweating like a bastard. That kind of stuff's happened to me about twenty times since I was a kid. I can't stand it (Salinger, 104).

While it is hard to determine whether or not Holden was actually molested by an adult, one cannot dismiss such a comment.