fizzle out
phrasal verb with fizzle verb [ I ] uk/ˈfɪz.əl/ us/ˈfɪz.əl/
逐渐终止，有始无终
to gradually end, often in a disappointing or weak way
They went to different universities and their relationship just fizzled out. 由于他们上的不是同一所大学，两人的关系便逐渐结束了。