This is an extremely difficult thing to do. Here are some ways - am not sure they work.
Think of the time when you were naughty at school and your teacher scolded you.
Think of when your mother scolded you for doing or not doing something.
Think of sad movies that made you cry.
Think of the time when your brother or sister ate up your portion of a delicious cake.
Think of the time when you lost out to your friend and you know that she cheated but nobody else does and you can't prove it.
Think about bitter medicines that were forced down your throat for various illness.
Think about vaccinations, injections, a dentist for sure!!!!!!!!!!!!
I can go on but I think I will stop here. I hope you don't start giggling when you read my answer. IF you do I will be very happy I made you giggle!!!!!!