Here are some possibilities of what you might be trying to say, along with corrected versions:
* "We've been apart since last [month/year/time period]." - This clarifies the time frame.
* "We've been apart since last [event]." - This clarifies what event caused the separation.
* "Have we been apart since last [time we saw each other]?" - This makes it a question asking for confirmation.
Please provide more information so I can help you write a complete and grammatically correct sentence.