Complex sentence: Marie-Thérèse, who was technically already queen, wore a gown covered in the royal fleur-de-lys; her hair was so thick that it was difficult to attach a crown to it.
Split into simpler sentences: Marie-Thérèse, who was technically already queen, wore a gown covered in the royal fleur-de-lys. Her uncovered hair proved to be so thick that it was difficult to attach a crown to it.