My mom took her life yesterday, I'm so confused.

Yesterday my mother took her life via a WWII rifle my grandpa had hidden in the basement. I have 2 other siblings, my older sister (age 19), my younger brother (age 10) and me (age 14). This morning my dad told us and I was distraught, im so confused and upset. What was going through her head? Didn’t she stop to rethink her decision, that me and my siblings would have been destroyed if she followed through? Every time I get upset I go to call her but I realize that shes gone and I cant, I miss my mommy so much. Why did she do this?