I trained in Japan, why was it so disappointing?

Simple answer first, your cultural programming greatly influenced the experience. Your cultural programming also influenced your expectations. More complex addendum is that the only way you can even begin to experience Japanese is to become one and that would take being born in Japan or at the very least to live in Japan for an entire lifetime.

Otherwise, even if you lived there for decades, your personal cultural programming will greatly influence that experience to a degree that you would and still are an "outsider" to the Japanese and the Japanese culture.

Many folks assume that to be a true martial artists one "must" travel to the place where it was born and to participate under the guidance of a Japanese Sensei and all that entails. Then we get these grandiose idea's that we will be certified by them and come back home as true Sensei of the traditional kind.

Often the well intentioned and anxious/excited visitor will be surprised and frustrated when that vision of exotic Asian martial arts turns out to not fit the picture, the assumptions, they created within their own minds.

So, the next question would naturally be, "how can I get the most out of my visit?" The answer is to study as much of the cultural system of the people of Japan, or Okinawa if your practicing karate, and learn as much about the culture, beliefs and cultural words, kanji and kana as possible then go with not expectations other than to train hard. Expect that the reality gap between you, a Westerner, and the Japanese Sensei, Senpai and Kohai of the dojo will be huge and work hard with humility. Especially, refrain your instinct and impulse to talk, talk, talk. Listen and Observe and when appropriate, not in the dojo but after when indulging in food and drink, ask our questions and actively/syntonically listen.

Have a good time and soak it all up, it is worth the time, effort and discipline.