Is there a polite way to decline a work party?

TL, DR: I am having a going away party planned by someone who doesn’t really know me. I am too shy and have PTSD and really don’t want a party. I don’t want to seem ungrateful or rude. Is there any way to decline?
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I will be leaving my job of 7+ years to move many states away to be a doctoral student. I have worked at this hospital for a total of 15+ years. During my time in this department, I have tremendously grown my position and have not only gained the trust and respect of many clinicians, but have also impacted clinical care and procedures through a variety of analyses and tracking of clinical outcomes.

I work almost completely independently and can go days without interacting with other people. I see my boss in the hallway every couple of months or so. We have no regularly scheduled meetings and he has taken on so much additional responsibility at work that I basically never ask him questions any more. There is another analyst and an administrative assistant with whom I interact every couple of months or so (I used to share an office with the AA, so we know each other very well). Otherwise, I basically have nothing to do with the rest of the division staff (mostly secretaries and research coordinators). Because of the divisive and frankly rude behavior of a few co-workers I stopped attending social work gatherings a couple of years ago.

I hear that someone in my department with whom I do not work is planning a work party. I very much wanted to avoid one. I do NOT like to be the center of attention and had hoped that perhaps my boss and I (and maybe a close co-worker or two) could eat lunch together as a going-away celebration.

Additionally, although most people at work do not know this, I have PTSD which has been triggered lately by the stress and unknowns of this upcoming move. I’m fine today, but it’s a bit unpredictable and I can’t always handle social interactions. At all. Like I can barely say hi to people some days.

Anyway, I am grateful that this coworker is thinking of me and wants to honor my leaving this job, but I absolutely do not want this party. And I don’t even know if I will be able to handle putting on a “happy face” on the day of the party.

Is there any way to get out of this??