since, for (time)
Used to express the time period from a certain point in the past until now.
Examples
彼と会って以来、ずっと幸せだ。
I have been happy since I met him.
日本に来て以来、たくさんの友人ができた。
Since coming to Japan, I have made many friends.
彼女は運動を始めて以来、体調が良くなった。
Her health has improved since she started exercising.
Quick check
彼と会って___、ずっと幸せだ。