I think you need to do something about this situation.  You need closure and you need to get it from the person who you feel hurt you.  That is your Stepson.  He is about to graduate... he is going to enter adulthood. He is plenty old enough to be able to explain himself.  You have blogged about this multiple times and lamented the fact that your husband can't/won't do anything about it.  So, I think it's time for you to bring the issue to a head with the person who is central to it and is old enough to answer for his own actions.

You need to ask SS why he did not invite you to the awards dinner.  You can tell him that you heard that there appeared to be room at the table and that it made you feel bad that he didn't feel you were worthy of an invitation.  Say you thought he knew how much you did for him and that your exclusion felt like a slap in the face.  That's it isn't fair to expect you to run around and plan things for him like his graduation party when you feel you aren't good enough to be included in things like the awards ceremony.

Again, unless you have spoken with the ORGANIZERS of this dinner, you don't know for certain that the kids were allowed to invite extra people.  The fact that there was an empty chair... and the fact that someone you knew thought you might be there.. doesn't mean that the kid was in fact able to invite more than 2 people.

Shoot, it might have been a misunderstanding with the boy.  But, at this point, you have gotten nowhere with your DH and it is festering inside you.  You are making up in your mind all sorts of nefarious reasons why you weren't included.. but the reality may not be that bad.  Or the kid may not really care.. but you need to ask the question to him directly.  Treat him like the almost adult that he is.  If HE made this decision.. HE can hear how he hurt someone's feelings and he can hear how you can choose to do LESS for him as a result.