Add something to track the length and allow you to do "checks" on whether the user is adding or subtracting text. This is currently untested but something similar to this should work (especially if you have a userform).
'add this to your userform or make it a static variable if it is not part of a userform
private oldLength as integer
Private Sub txtBoxBDayHim_Change()
if ( oldlength > txboxbdayhim.textlength ) then
oldlength =txtBoxBDayHim.textlength
exit sub
end if
If txtBoxBDayHim.TextLength = 2 or txtBoxBDayHim.TextLength = 5 then
txtBoxBDayHim.Text = txtBoxBDayHim.Text + "/"
end if
oldlength =txtBoxBDayHim.textlength
End Sub