Blame someone else :D
Seriously though, try writing in more detail about why you did it.
If you have a straightforward reason - illness, bad future, etc. it won't be as bad as it might seem.
Especially if that reason is known to everyone.
I myself have thought about how to minimize the suffering of loved ones, and I don't know what to do.
I'm afraid that if I write everything in detail and honestly, it might intensify my parents' suicidal thoughts.
But if I write too little, they might blame themselves, and that would also worsen their suicidal thoughts.
The problem is that my parents still have underage children who need them.
I even thought about faking a simple reason.
Like depression - writing the letter in a way that makes it clear the person who wrote it was depressed.
Writing that I stopped enjoying life, that I often just lay in bed and didn't want to do anything, etc.
But even that might lead to guilt, because they didn't notice this reason.
So it's a very difficult issue.
At the moment, I'm leaning towards writing two letters - one short one that they receive after CTB, and a second one they receive 2–6 months later, where I explain everything honestly and in detail (currently it's about 9,000 characters), but with a disclaimer at the beginning (as a text file on a flash drive) that says: if you have or had suicidal thoughts, don't read further. The reason for the death will become clear after reading, but it will only make things worse.
Try reading forums (like r/SuicideBereavement), where people share how they coped with a loved one's CTB and what they think/feel.
After reading, I added some questions from those who lost loved ones to my letter and answered them.