WPF TextBox text selection issue - wpf

Question

I have an issue with selecting part of the text in TextBox. I cannot understand why, I can select just one character at a time and I can select as many as I want with the keyboard (shift+arrow). This is my code:
<Style TargetType="{x:Type local:EditableTextBlock}">
<Setter Property="FocusVisualStyle"
Value="{x:Null}" />
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type local:EditableTextBlock}">
<Grid Background="{TemplateBinding Background}">
<Grid.Resources>
<BooleanToVisibilityConverter x:Key="bool2Vis" />
<local:BooleanToInverseVisibilityConverter x:Key="bool2InverseVis" />
</Grid.Resources>
<TextBox x:Name="textbox"
Visibility="{Binding IsInEditMode, RelativeSource={RelativeSource Mode=TemplatedParent}, Converter={StaticResource bool2Vis}}"
TextWrapping="{TemplateBinding TextWrapping}"
Text="{Binding Text, Mode=TwoWay, UpdateSourceTrigger=LostFocus, RelativeSource={RelativeSource Mode=TemplatedParent}}" IsHitTestVisible="True" Focusable="True"/>
<ContentControl x:Name="textblockContainer"
Visibility="{Binding IsInEditMode, RelativeSource={RelativeSource Mode=TemplatedParent}, Converter={StaticResource bool2InverseVis}}">
<TextBlock x:Name="textblock"
Text="{TemplateBinding Text}"
TextWrapping="{TemplateBinding TextWrapping}"
TextTrimming="CharacterEllipsis"/>
</ContentControl>
</Grid>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
Any help will be appreciated.
Thanks in advance.