What chapter in Sailor Moon manga do darien and usagi break up?

Darien and Usagi don't actually break up in the Sailor Moon manga. They do have a very difficult period in their relationship during the Death Busters arc (Act 49-51), where Darien forgets his memories of Usagi due to the power of the Death Busters. This leads to a lot of emotional pain for Usagi and a lot of misunderstandings between them.

However, they eventually remember their love for each other and reconcile, becoming stronger than ever.