I think most people go for 'sorry'. That you're grateful for all they did for you. That you know they would have helped if they could but no one could help type thing. I don't know how long you've been suicidal for but that's something I want to put in- that it was a wish I'd had for a very long time and I hung on for as long as I could. So- it wasn't an impulsive act. I think it would be good to try and explain why. It's probably true that words are never going to be enough but I think it's better than nothing.
Someone posted this the other day- to try and work out what questions they might try and answer in a note- maybe it might help:
i would like to include every answer possible in a detailed “side” note for my family. i know what it’s like to be left with questions, how destructive and festering that feeling is, and i would like to prevent that after my death if possible.
sanctioned-suicide.net