You won't get chance to rest because it will be you doing the work to get the faculty out of a hole.
It should be her health that is worse than yours - it seems like she has less stress which is wrong. The Dean gets paid to be stressed. Why did the Dean let others quit? She knows you the only one who can do the work.
You need to write to the President explaining the situation, attach a doctors letter about your health. If you want to quit you quit. You must be strong. It is up to you to force this because this Dean will not let you go easy because she knows she will find it hard to replace you. Mention to President Somchai in your letter you tried to quit already.