Sorry for your loss, and sorry it’s a complicated one 
Theres a few very difficult people in my family and one of them died recently. His daughter acknowledged the difficulty in her reading which was good, as there was an awful lot unsaid at the funeral and I think it was good to acknowledge the hurt this person had caused.
This kind of thing might be a good option? But there’s more if you google - I searched for “poem for funeral of a difficult mother” and there’s quite a few articles and ideas around to browse.
‘A Bird in Mist’ by Anonymous
At times like this,
We may look through books for the perfect words
To give form to our feelings,
Make the thing complete,
Set the matter at rest.
Yet, in hours of searching,
Each piece lies rejected – too precise, too difficult, too sweet,
Too harsh, implying what we do not wish to say.
But, look into the grey wide sky,
And the thoughts will come like this:
Remember me when I loved you most and you loved me most.
Remember me when I was my bravest and I did you right.
Then, let that be our secret bond
And, let us rise in the morning and enjoy the light
And know that the bird in the mist is returning to the sun.
juliemuircelebrant.com.au/blog/funeral-poems-readings-difficult-people/