Is there a way to improve my grade after graduation?
There's nothing you can do to change your grade on a completed degree.  What you can do is do well in your job so that no one cares about your old grades, which it sounds like you've done.  So congrats on that!  The real issue here isn't your grades, it's that you're still feeling a lot of anxiety about your college experience.  I'd suggest talking to a therapist about that.  You can't change your grade, but you can better understand the feelings you're having about your college experience, and you can change how you think about those grades.  I wish you all the best.