What is the difference between this two sentences?
We're in love and have been for eight year.
We're in love and were for eight year.
Your first sentence should be:
We are in love, and we have been for eight years. ["have been" is Present Perfect and it means an action (your falling in love with each other) began in the past and continues to the present (you both are still in love with each other)]
The second sentence should be:
We were in love for eight years. [were is past tense (I was, they were, we were,etc.) and the past tense means an action started and ended before now. After eight years, your love for each other ended.]