It sounds like this is partially caused by your DM having a simplistic understanding of LG types. (no offense)
Why would the monk be expected, in any way shape or form, to Forgive & Forget?
Action dictates alignment, not vise-versa. The more true the player plays to his character, not some abstract morality system, the better.
I couldn't disagree more. First, alignment is temporally primary in that we must select one before we can use spells etc. Alignment is selected before any action is ever taken in the game. It will inevitably condition all future action. Second, alignment is logically primary to action. Action derives from a character's motivations, those motivations are a product of his experiences and desires. Both are heavily conditioned by alignment in a fundamental way, especially desires.
Based on these definitions, being Lawful Good in itself would make you neither more nor less likely to forgive and forget. A Lawful Good character would certainly want to ensure that justice is done, and conducted according to due legal process. Beyond that, it is really up to the character. One may choose not to associate with the offending character any longer, while another may choose to try and reach out to him and rehabilitate him.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.