High school, for me, was also a hideous experience, albeit for significantly different reasons. I basically just hated the world at the time, and felt like the world hated me.
I am approaching my 10-year reunion next year and I have already made the decision that I won't be going. Similarly to you, I keep in touch with the only ones who truly mattered to me. Other than that, I don't care to see the rest or to revisit those old memories and faces [i was bullied quite a lot].
I agree that if you nearly vomited just by opening the letter, then perhaps this is one battle you can sit on the sidelines for and just not worry about it. The choice is eventually yours. I think in this case you need to decide if the possible benefits outweigh the possible risks. Sounds to me like that isn't the case here.
All the best,