I'm not sure there is anything you can say in such a note that would be comforting to your parents, at least not for several years and much therapy after the fact at best.
You could write a novel on all of the reasons that it is not their fault, that they were good parents that showered you with love, that you deliberately hid your pain from them and that this couldn't have been prevented by them being more observant or checking in more closely, and they would still very, very likely find some way to blame themselves or wonder what more they could have done.
On a pragmatic level, it may be helpful to leave key username and passwords, bank account info, ect. accessible to them so that the logistics of processing your post-mortem affairs is easier (someone wrote a lovely post that I'll try to dig for).
Found it;
Thread 'How to Leave with Minimal Pain for Those Who Remain'
https://sanctioned-suicide.net/threads/how-to-leave-with-minimal-pain-for-those-who-remain.207480/