I would encourage you to go to the promotion board. I had numerous Soldiers tell me they were getting out and did not want to go to the board and then ended up staying in and they burnt the bridge with their leadership. Getting out as a SGT (P) or SSG looks better on your resume then getting out as a SGT.
If you really want to get out of it go to the board and if you get selected you should be able to turn down the promotion in writing. See AR 600-8-19 for specifics.
Regardless of how you feel now, I would encourage you to go to the board. There are a lot positive things associated with going to the board and getting promoted. Don’t let how or why your leaders select you for the board interfere with how you excel in life and your career field.