I tend to write a huge sprawl of everything I want to say in a really messy order, and keep adding to it, somtimes over days until I've got everything down. Then I just try to join them up into sentences then paragraphs.
Personally I would straight out say...
i have no idea how to finish the letter. actually finishing it feels too much like drawing a line under it.
i have no idea how i will manage without him because he is genuinely amazing...but how do you thank someone for being that awesome???
Just possibly without the three question marks
It's great that he's stopping you from messing up. Keep a hold of that, even when he's gone.