What are the things or traits Holden needs to fix about himself?

Holden indeed has some challenging sides to his personality, but whether or not they need "fixing" is a debatable point. He has unresolved grief issues around the death of his brother, which leads him to act out in some difficult ways, and he also may be dealing with the trauma of having been abused by Mr. Antolini. Both of these circumstances may have something to do with Holden's most difficult traits, which means he could benefit a great deal from some sort of therapeutic treatment, if that is what is meant by "fixing."
Holden certainly has an erratic side to his personality that can be jarring and difficult. For example, his relationship with Sally seems to inspire significant emotional swings. One moment, he is gentle, telling Sally he loves her, and the next, he is complaining about her and contradicting his own positive comments about her.
Also, Holden also has an intensely negative view of humanity, which means that most people he encounters bother him, annoy him, or get under his skin in some way. This pessimism combined with his sensitive manner makes for some challenging situations.
Also, Holden can become loud and aggressive with very little warning, which can inspire fear and anxiety in others. This volatility may be the result of the unresolved emotional issues in his life that reach back before the start of the book. The trauma of grief and abuse can combine to have a terrible effect on a person, and Holden is young and somewhat isolated from his parents, which means he has to cope with these challenges on his own.