But the Marines is [insert word here] so that's why I'm glad I joined.
I am sorry but but I am unable to compress all your requirements into one small package and the presentation had to be adjusted a little but I think this fits in with the context of your conversation. However I have appended what I think is a better solution.
Jimmy: I'm proud to have been a United States Marine.
Sally: But you hated being in the Marines! All your letters talked about how much you regretted joining.
Jimmy: Well, yes, those were my feelings at the time. But being a Marine "was a wistfully defining exercise", so that's why I'm glad I joined.
Examples
He speaks wistfully of his time in the limelight.
Responsibility, I suppose, is what defines adulthood.
an exercise in public relations
Alternative
However if it was my choice I would not try to convey the feeling of Nostalgia into the spoken statement but rather in how it is delivered.
Jimmy: I'm proud to have been a United States Marine.
Sally: But you hated being in the Marines! All your letters talked about how much you regretted joining.
Jimmy: Well, yes, those were my feelings at the time. But being a Marine "defines you ", so that's why I'm glad I joined.; He replied wistfully
wistfully: adverb: in a way that is sad and shows someone is thinking about something that is impossible or in the past:
define verb [ T ] (EXPLAIN) to explain and describe the meaning and exact limits of something:
exercise noun (PRACTICE) an action or actions intended to improve something or make something happen
All Definitions and Examples supplied by Cambridge English Dictionary