I recently got my SAT score back and got a 1570 composite, which completely exceeded my expectations. However, I got a 15 on the essay portion, which really shocked me. I don’t plan on retaking the test unless absolutely necessary–I’m aiming for schools like the Ivy Leagues, Stanford, UChicago, etc.
I got a 5 on the AP Language Exam, so I was definitely expecting a higher essay score.
Am I screwed? 
No, of course you are not. 1570 is an impressive score. That 15 is just a tiny piece of your application.
I don’t see it reasonable to take the SAT again to fix the essay score…my opinion…English grades through HS and AP tests are more important.