Textbook vs Real
Textbook보고 싶었습니다bo-go si-peot-seum-ni-da
Real보고 싶었어bo-go si-peoss-eo
Why Koreans say it this way
The most common way to say "I missed you" in real Korean. ~었어 (casual past) instead of ~었습니다 (formal past). No one says the formal version to someone they love.
From: 도깨비 EP13