Dear {Name},
I am so sorry for leaving your party without saying goodbye. I'd like to make the excuse that {crowd/you were busy/etc.}, but honestly I just bailed.
As much as I appreciated your invitation, once I showed up, I realized {social anxiety/ex was there/other mitigating factor}. You are an amazing host, and at the very least you deserved an explanation that night.
I hope that you will forgive my rudeness. I always enjoy your get-togethers and I hope my poor behavior doesn't get me crossed off your guest list!
Sincerely,
{Name}
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