Unfortunately, I don't know how your list box is set up. Is it bound or not, what is the control source. Number of items in the list, how the data is getting there etc.
Have you considered using a combo box instead. Check help to see the differences. Type in list box and look for Should I create a list box, a combo box, or a drop-down list box?
Hopefully, after reading this, it will give you some ideas.
HTH
An investment in knowledge always pays the best dividends.
by Benjamin Franklin