Another thought, too.
Stop looking at the bike computer when you are starting to feel "negative". I know that it's needed for intermediate distances, but if it's a long stretch, there is nothing worse than thinking 10 minutes has gone by and looking down to see you've only done one mile.
Time just does seem to defy itself in those circumstances.
Of course, noting that your speed really is 6 miles an hour -- on the flats -- might lead you to conclude that you are on the way to bonking.