Should a non-discretionary (pro-rated) bonus, based solely on my performance, be paid upon leaving?

I receive a base salary and an incentive bonus (20% of collections of everything over a set threshold, based solely upon my performance). Some people I work with just receive a percentage of collections (ranging from 30-50%). Therefore, I feel that my "bonus" is non-discretionary and truly part of my salary, just a smaller percentage of collections given that I have a cushion (base salary). My bonus is given once a year. I will be just over 1/3 of the year into my bonus when I leave and I feel the bonus should be prorated, however, my boss states I have to be there the full year to receive the bonus. Unfortunately, I was INCREDIBLY stupid to not realize or think about having something in my contract stating it should be prorated until it was too late. While my contract states it is given once yearly, it does not specifically say what happens when I leave part way through the year. Do you have any idea if this is something I would be entitled to or do you think I'm out of luck?