No matter why your relationship ended, your Pisces ex will always take your breakup very seriously and personally. In fact, if it were for them to decide, Pisceans would sweat their guts out to make your relationship last; they would even try to reanimate it at all costs.
Your role in this mess is not to let your Pisces ex waste their effort in vain and to explain to them it is better to move on and continue to communicate as friends. Since your being together may have been their dream, you need to think of something to shift their focus onto. In other words, help your Pisces ex find a new dream.