Guilt about buying three tickets instead of four is not the same as wanting to stop the divorce. I know it is hard to keep the two separate in your mind. Even though you say you have made the decision, it must still be scary to go through the process.

If you had bought 4 tickets and allowed him to come, would that have put you in a triggering situation? (Is it likely that he would have upset your evening?) If yes, then maybe three tickets was the right decision at this time.

Though even if you divorce there will be times all of you will need to participate in things together; but maybe you have to get the divorce behind you before you are emotionally ready to handle that? As far as your divorce goes, I have decided I am unable to give you logical, unbiased advice. I want you to do what is best for you and would think that you are just as cool as a divorcee as you are married.