I would like to replace a string with the a variable which contains newlines in it. However, sed replacing doesn’t seem to do what I want.

1. What I am doing now

    str=""
    str+="#My header 1"$'n'
    str+="#My header 2"$'n'
    str+="#My header 3"$'n'
    echo "$str"
    #My header 1
    #My header 2
    #My header 3
    
    sed "s/##HEADER##/${str}/g" myfile.out

Unfortunately this does not work obtaining the following error:

sed: -e expression #1, char 25: unterminated s command

Any other ideas?