Nick means that the experiences of the past five years have taught him to see things as they really are. He will no longer rationalize and make excuses for himself, allowing himself to be swept into the corrupt and immoral lifestyle of the people he has encountered in the East. He is thoroughly disillusioned by his "friends'" behavior - when he is honest with himself he knows "they (are) careless people...they (Smash) up things and creatures and then (retreat)...and let other people clean up the mess". Nick says, "After Gatsby's death the East was haunted for me...distorted beyond my eyes' power of correction...so...I decided to come back home".
Nick's quote is directed at Jordan Baker. Although he has decried her shallowness and amorality throughout the novel, he allowed himself to be drawn into her way of life by having an affair with her, and even in Chapter 9 her allure for him is such that he briefly wonder(s) if (he isn't) making a mistake" in letting her go. After thinking about it for a minute though, he sticks to his resolve to leave behind all the decadence and corruption, because he is "five years too old to lie to (him)self and call it honor" (Chapter 9).