it gets removed because of the way you are building your string.
low=1:Lvalue;
low1=num2str(low');
high=Value:50;
high1=num2str(high');
new = [low1 editbox high1];
you are always setting the strings before and after your selection to be 1:Lvalue and value:50.
I would suggest doing this:
Add editbox string directly below the selection.
function add_Callback(hObject, eventdata, handles)
editbox = get(handles.edit,{'string'}); %need {} because editbox is a char
Lstring = get(handles.listbox,'string');
Lvalue = get(handles.listbox,'value');
% update the string in the selected place with the editbox data
%Lstring(Lvalue)=[editbox]
new = [Lstring(1:Lvalue) editbox Lstring(Lvalue)];
Value = Lvalue+1
set(handles.listbox,'string',new,'value',Value);
let me know if this works for you
