What is the best way to express regret at something that didn't happen? (informal)


I'm trying to write a postcard to a Japanese friend that I didn't manage to
meet the last time I visited. I'm not sure if I should start the sentence with
something like 残念ながら or 残念なことに (and in that case, if there is some difference
in register and meaning I should be aware of), or if there is a way to use a
negative verb together with "te-form しまいました" (I don't think there is, but
maybe there is another similar verb that works with negatives?).

