Validated Textbox in a UserControl

I have created a UserControl - a Labeled TextBox which is working pretty well, except for the validation template. When there's an error the validation control template shows up but it fills the whole space including the Label. I only want it to be as big as the TextBox. How to fix this?

Here's the xaml:

<UserControl x:Class="Infrastructure.CustomControls.LabelTextBox"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             x:Name="LTB">

    <Grid HorizontalAlignment="{Binding}">
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="Auto"/>
            <ColumnDefinition Width="*"/>
        </Grid.ColumnDefinitions>

        <TextBlock x:Name="tbl" 
                   FontFamily="{Binding}" 
                   FontSize="{Binding}" 
                   Text="{Binding ElementName=LTB, Path=LabelText}" 
                   Height="{Binding ElementName=LTB, Path=LabelHeight}" 
                   Width="{Binding ElementName=LTB, Path=LabelWidth}" 
                   VerticalAlignment="Center"/>
        <TextBox x:Name="tbx" 
                 Grid.Column="1" 
                 FontFamily="{Binding}" 
                 FontSize="{Binding}" 
                 IsReadOnly="{Binding ElementName=LTB, Path=IsReadOnly}" 
                 MaxLength="{Binding ElementName=LTB, Path=TextMaxLength}" 
                 Text="{Binding ElementName=LTB, Path=Text}" 
                 Height="{Binding ElementName=LTB, Path=TextHeight}"
                 Width="{Binding ElementName=LTB, Path=TextWidth}" 
                 VerticalAlignment="Center">
            <Validation.ErrorTemplate>
                <ControlTemplate>
                    <DockPanel LastChildFill="True" 
                               ToolTip="{Binding ElementName=aep, Path=AdornedElement.(Validation.Errors)[0].ErrorContent}">
                        <TextBlock DockPanel.Dock="Right" 
                                   Foreground="Red" 
                                   FontSize="14pt" Text="*" 
                                   Margin="-15,0,0,0" 
                                   FontWeight="Bold"/>
                        <Border BorderBrush="Red" BorderThickness="1">
                            <AdornedElementPlaceholder Name="aep"/>
                        </Border>
                    </DockPanel>
                </ControlTemplate>
            </Validation.ErrorTemplate>
        </TextBox>
    </Grid>
</UserControl>

Thanks for the response, it helped me figure out where the problem is. And for that i vote it as useful.

What i did was programmatically query the UserControl validation for errors in the TextBoxChangedEvent and set the validation error manually (http://wpftutorial.net/ValidationErrorByCode.html) for the TextBox.