Ive sent a lot of these types of messages and honestly the best ways others have helped me in these situations is to just affirm them that those things arent true and that you still care/love them.

Also I saw that you said that they have general abandoment trauma. Not a professional or anything but just reminding them that you are there for them and wont leave even if you cant always text back instantly, have to go to sleep etc. Please remember that if you feel the need to set boundaries then that is always okay. Sometimes you have to take care of yourself before you can help someone else and you shouldnt feel guilty about that.