Why was Saul's realization on the Damascus road so shattering, and what happened to him as a result?

Saul had been serving Jehovah while trying to stamp out Jesus, so when the heavenly voice said "I am Jesus," he realized Jehovah and Jesus are one and the same. The realization blew his mind, and as a result he ended up blind.

Les explains that this is Saul's conversion account in Acts chapter 9, written by Luke. A light from heaven shone around Saul; he fell to the earth and heard a voice say, "Saul, Saul, why persecutest thou me?" Because the pronoun "me" came from heaven, Saul understood it must be God speaking. Saul answered, "Who art thou Lord?" The word Lord stood in Jewish vocabulary for Jehovah, which they would not voice; so in his mind he was saying Jehovah while his lips said Lord. Jehovah answered, "I'm Jesus." Since Saul had been serving Jehovah and trying to stamp out Jesus, the sudden realization that they are one and the same blew his mind and left him blind.

Acts 9:8

And Saul arose from the earth; and when his eyes were opened, he saw no man: but they led him by the hand, and brought him into Damascus.

Acts 9:9

And he was three days without sight, and neither did eat nor drink.

What realization did Saul have on the road to Damascus about Jehovah and Jesus?

On the Damascus road, Saul learned that the Jehovah he served and the Jesus he persecuted were one and the same. Addressing the heavenly voice as Lord—the term used for Jehovah—he heard the reply, I'm Jesus. This revelation stunned him and left him blind.