Combobox selecteditem text trim

im having problems with trimming on a combobox selected item.

i managed to get the items in the dropdown to have an ellipps but not the selected item.

the code i have in the style of the combobox it 

<ContentPresenter x:Name="ContentPresenter" HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}" Margin="{TemplateBinding Padding}" VerticalAlignment="{TemplateBinding VerticalContentAlignment}"><TextBlock x:Name="textBlock" Text="" Margin="-10" Padding="0,0,0,5" VerticalAlignment="Stretch" HorizontalAlignment="Stretch" TextTrimming="WordEllipsis" TextWrapping="NoWrap"><TextBlock.Foreground><SolidColorBrush Color="{StaticResource DropdownTextColour}"/></TextBlock.Foreground></TextBlock></ContentPresenter>

This code isnt working 

Any Advice?