OK, time for some more pain.
A clothes peg on each nipple. Make sure you get it to clasp right on the tip.
Now keep stroking until the bite on your nipples begins to subside.
As soon as you think you're ready to carry on, twist both pegs 90 degrees and hold a 5 seconds. Then carry on stroking until you've got used to the pain again.
Get used to the feeling - it's going to be with you for quite a while...