Search from a specific place in my listbox?

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C#

Hello everyone! I need some help with my listbox in an application i've done. First of all i have 2 variables, one that keeps count of how many items there are in the listbox and another that counts how many "Kills i've done in one trip" (the program is basically made for a game where i log all the drops i get from a specific boss). I have a searchfunction where i can search for any item and calculate the rate of getting the drop etc. But i have a button which resets the "Kills this trip" variable to zero aswell and I would like the searchfunction to search for only those items that i've got on that trip when i check a checkbox (Next time i add an item, the variables for "kills this trip" will increment with 1 again). Let's say i've done 100 kills in total and then reset "kills this trip" to zero, then I just want the search function to search for items after the 100th item. This is how my searchfunction code looks like:

string searchString = textBox1.Text; //searchstring for the searchfunction

           if (searchString.Equals("")) //checks that the textfield isn't empty
           {
               MessageBox.Show("Enter an item!", "Obs");
           }
           else
           {

               for (int i = 0; i < droplog.Items.Count; i++)
               {
                   if (droplog.Items[i].ToString().IndexOf(searchString, StringComparison.OrdinalIgnoreCase) >= 0)
                   {
                       droplog.SetSelected(i, true);
                   }
                   else
                   {
                       // Do this if you want to select in the ListBox only the results of the latest search.
                       droplog.SetSelected(i, false);
                   }
               }