It is understandable to view a tantrum as an irrational inconvenience that needs to be eliminated as soon as possible. But parents whose children thrived saw outbursts as teachable moments and a time to bond with their children. Yes, that does not always feel natural when a child falls to the ground screaming and angry.
Does something like "You shouldn't feel this way" ever work with emotionally intense adults? No. Then it sure won't work with your son either. Saying "There is nothing to fear" or "Oh, everything will be fine" is derogatory. This is how children learn to doubt their own judgment and lose confidence.
A tantrum is the best time to connect with your children and teach them a valuable skill. Yes, you must stop the bad behavior immediately. But it is better to do it in a way specific to the child's actions and not to do it on his identity. So you mean, "We didn't paint Grandma's couch purple," instead of, "Stop being a nightmare!" Children who constantly heard the latter did not do as well Emotional Intelligence.
When children experience disrespectful and dismissive behavior from their parents, they have more trouble with school work and don't get along with friends. They have higher levels of stress-related hormones in their bodies. They tend to have more behavior problems at school and they get sick more often from high cortisol in their body.
It takes practice, but you have to view children's emotional pain as you would physical pain. It is not your fault. It is a challenge they face. And one that you can help them with. Become an emotional mentor, not a corrections officer. Your children need you!