What to tell my boss 😕

Sara • Married to my highschool sweetheart. SAHM of 2 wildflowers.
So, Tuesday I am telling my boss I am quitting the first week of December. I am the only person in my office who knows how to do my job so I know she is going to flip her wig! I feel like I am going to make up some really good excuse why I need to leave when I am actually just doing it due to major work stress and to have time to have prepare for our baby girl. I know that will not be an acceptable answer for her. I'm a nervous wreck. I am asking for prayers and any advice. Thank you in advance.