I get this. It is ok, you don't have to go. It sounds like it would be intensely triggering for you, and it's a coworker, not a close friend or family member.
I went to a coworker's brother's funeral since Squish died, and boy, was it a mistake. Though I worked in a small office, and she was in my department, there was no good reason that I should have gone. It just made me sad for the rest of the day and week.