I had the same experience whilst a flight was descending - I was in so much pain, and I thought my eyeball was going to explode, and I actually thought I was going to die! I got some pretty strange looks from other passengers due to my near hysteria.....
I had contracted a sinus infection whilst I was on holiday, and this was the cause of the pain.
I have found a solution - there are some earplugs that you can buy from the chemist, which have a hole through them with a small valve inside. The valve helps to equalise the pressure inside your ear/eustachian tube. I live in the UK and got mine from Boots Chemist - but I am sure thay must be sold worldwide.
I never fly without them - you only need to wear them when ascending/descending or if the pressure changes in the cabin.
Hope this helps