How can fix Dynamically load usercontrol postback problem?

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ASP.NET

I have four listbox some categories for ex otomobile->opel->astra->1.6 etc.
every selection has listbox and changes listbox select index changing event.
first select main categories = otomobile or housing or jobs etc.
if select main categories I will load usercontrol because every main categories usercontrols different.

for ex I selected otomobile category otomobile usercontrol dynamically load inside contentplaceholder panel.

Control login = LoadControl("UsrCntrl/otomobile.ascx");


                if (Session["xxx"] != null)
                {


                    Panel1.Controls.Add(login);


                }

these also source code

<asp:Content ID="Content6" ContentPlaceHolderID="CntHeader" runat="server">

         <div class="product-filter">
         <div class="display">Kategori Seç</div>
          </div>

          <table style="width: 100%">
                     <tr>
                         <td>
                          <asp:ListBox ID="ListBox1" runat="server" Height="150px" Width="200px" AutoPostBack="True"
                                onselectedindexchanged="ListBox1_SelectedIndexChanged"></asp:ListBox>

                         </td>
                         <td>
                          <asp:ListBox ID="ListBox2" runat="server" Height="150px" Width="200px"
                                AutoPostBack="True" onselectedindexchanged="ListBox2_SelectedIndexChanged"></asp:ListBox>

                         </td>
                         <td>


                          <asp:ListBox ID="ListBox3" runat="server" Height="150px" Width="200px"
                                 AutoPostBack="True" onselectedindexchanged="ListBox3_SelectedIndexChanged" ></asp:ListBox>

                           </td>
                            <td>

                            <asp:ListBox ID="ListBox4" runat="server" Height="150px" Width="200px"
                                    AutoPostBack="True" onselectedindexchanged="ListBox4_SelectedIndexChanged" ></asp:ListBox>


                         </td>
                         <td>
                             &nbsp;</td>
                         <td>
                             &nbsp;</td>
                     </tr>
          </table>
</asp:Content>

<asp:Content ID="Content7" ContentPlaceHolderID="CntIcerik" runat="server">


    <asp:Panel ID="Panel1" runat="server">
    </asp:Panel>








</asp:Content>

otomobile usercontrol has a country dropdownlist autopostback true
if I change data from dropdownlist another dropdownlist fill cities
for ex I select germany another dropdown fill cities frankfurt,bochum etc.

this is the problem here,if I select country page postback running so usercontrol lost.

how can I fix this problem?