We have thought it sad

It looks like a incomplete sentence.
I guess it should be either "We have thought it said love" or "We have thought it is sad love",if it is the first one,then it can be translated in "我原本认为它说过爱",and "我原本认为那是悲伤的爱情”for the second one.
Hope you enjoy.

我们曾经认为那是哀伤的爱，这个要根据上下文才好翻译，

我们曾经认为其为苦情。

我们认为其是苦情~