Phrase for something that isn't as good as it once was [closed]

Specifically a relationship which fell apart as time wore on.

A common expression is to lose the spark in a relashionship: ( from TFD)

• vivacity, enthusiasm, or humour

• 5 Reasons Your Relationship Has Lost Its Spark.

• Why the Spark Fades in a Relationship.

• Countless couples complain of losing the “spark” in their relationship. Some chalk it up to evolved differences, a slow growing apart, or sheer familiarity. The wave of “deadness” that can submerge a relationship after the first thrilling months or years have caused many couples to lose hope, and even look elsewhere for the excitement of newfound intimacy.

You could say that the relationship has grown stale. Dictionary.com:

4. having lost freshness, vigor, quick intelligence, initiative, or the like, as from overstrain, boredom, or surfeit: He had grown stale on the job and needed a long vacation.

"Lost its lustre" is a nice way of putting it.

Lustre - 1A gentle sheen or soft glow. 1.2 Glory or distinction.

http://www.oxforddictionaries.com/definition/english/lustre

"We grew apart" is a more relationship-specific phrase.