Waxing and waning are perfectly good general verbs which have been in use for 400 years at a guess.

When love wanes, you can say, poetically that it is "growing" cold even though it isn't growing but shirnking (different sense of the word, i know) and you can likewise say poetically that it is waxing cold, which should not be confused with f***ing cold, a different construct altogether.

A