I agree with don't go. Putting yourself in the Not Going camp...You feel some relief? Yes? That is an indication it is the right answer, imho.
On guilt: you are setting boundaries for yourself because of their behavior.
(People set boundaries for themselves based on others' behavior all the time and not due to horrid behavior-just an off comment can justify it...And I don't think guilt enters into it in those circumstances at all, iyswim.) I think that you are making yourself feel guilty. You don't have to. Just stop it.
None of this was your fault. As previously said, you do not owe them anything. Nothing. Nada. Null set. Zero. Zip.
Check again: nope, still don't. And there is no guilt to attach to that.
15 months not seeing them is fantastic!! Congratulations! Imho, going will be a huge step backward. The anxiety you feel is a negative (as opposed to looking forward to an event with pleasant anticipation) and I think that is the gut feeling/guardian angels/subconscious guiding you to decline the event.
You do not sound like an idiot at all. It is hard to disengage, disentangle yourself from things engrained in childhood (brainwashing, if you'll forgive me).
I doubt an announcement of your position and boundaries will be respected by them. It will just be another opportunity for them, your step father especially, to be dismissive, disrespectful, and ridicule you...in the moment and for many years to come. Do not hand them that ammunition.
Let the hostess know you won't be there so she/he will not cook extra. You do not need to give a reason. "It is just not possible at this time" is a generic statement and if folks are rude enough to press beyond that then that is their problem and you do not need to answer...(It's complicated, and you just don't want to talk about it).