1. To make convulsive efforts; to struggle; to strive in physical exercise: The hikers were agonising in the hot sun as they were climbing up the high mountain.
2. To be in agony; to be in great pain: Susan agonized over the difficult decision that had to be made before making the final phone call.

Regarding something that causes mental or physical pain: Lynn thought that labor pains were the most agonizing kind of physical suffering she could remember, but they disappeared and were gone as soon as the baby arrived!