My biggest problem is that I there are times when I need to be aggressive but seem to be incapable of generating aggression. Even guys who I have trained with have noted this. How can I become more aggressive?
Are you performing well, without another level of aggression?
Bjorn Borg & John McEnroe both has diametrically opposing personalities, both were the best tennis players of their era and fierce rivals and competitors.
Everyone has aggression when they a reason to be aggressive.
You need to discover that key, set objectives, directives, goals...
Everyone reacts/responds uniquely based on their make-up.