writing a difficult letter

Possession - Section 8 and Section 21

I have a very difficult letter to write.
I have the ground floor and basement flat of a building, with 1 other flat above.
The ground floor tenant has been with us since 2008 and has developed many health issues, including mental health. She has sworn at and abused a series of basement tenants so much that the agent of the basement flat is now saying he can't let it as it has been put in writing (they don't manage the ground floor one, so I can't ask them)
The time has come to give her notice and I'm struggling with how to write the letter (not the Section 21 part, the part where I'm giving my reasons). Having known her for so many years, it's not appropriate to just send a section 21 on its own
She is currently going through multiple health issues (this is a permanent state of affairs, so I can't wait until it has passed) and is also supporting her ex who has received a terminal diagnosis
I feel terrible about having to ask her to leave, but we've tolerated her upsetting her neighbours for several years now, and this is now crunch time
Does anyone know anyone who can help me write the letter?
I want to hide under the desk!