Spotkałem (masculine form of "I met"). If a woman were saying "when I met you" to a man, it would be: "Kochanie, kiedy Cię spotkałam ."
Focus on how your life changed from "before" to "after." kochana_kiedy_cie_spotkalem
If you are writing to someone special, you can focus on the "first moment" impact. Spotkałem (masculine form of "I met")
The phrase (My dear, when I met you) sounds like the opening line of a romantic poem, a sentimental letter, or a nostalgic song lyric. it would be: "Kochanie
"Kochana, kiedy Cię spotkałem, świat nagle nabrał kolorów, o których istnieniu zapomniałem." (My dear, when I met you, the world suddenly took on colors I had forgotten existed.)