Joint Guardianship application with difficult ex

My daughter will be 16 (adult) in a years time and due to her disability and lack of capacity a welfare and financial Guardianship application will be suitable and required.
Daughter lives with her mother who is my ex-spouse and daughter has overnight contact with me every weekend and mid-week time too (court ordered).
Her mother has struggled to interact with me since the break up and communication with her is always negative and frustrating. I do not trust her to always act in my child''s best interests either in relation to welfare or financial decisions (she has paid off her mortgage early with my daughter''s benefits but sends her to me dressed in rags).
When I have discussed a joint Guardianship application with social work they portrayed it as going to be difficult due to the poor trust and poor relationship with his mother. They implied her mother being the only guardian would be expected.
I am looking for some advice about how I might best look at setting the groundwork for a joint application as I think my child having myself as joint gauradian will be necessary for all aspects of her wellbeing. All advice gratefully appreciated. Thanks .

Name of child removed by Wikivorce Moderator