ok I had a problem with this to but its actually quite easy. HOW YOU FEEL AND WHERE YOU ARE IS WHEN YOU USE THE VERB ESTAR.
so for example : bella esta contentas - bella feels happy, if you were to say bella es contentas - bella is happy you would basically be saying bella is a happy person as in that's a permanent thing . I hope I helped
you use ser when you are describing, for example: Maria es alta, Maria is tall. Also when you are indicating relationships, el es mi amigo, he is my friend. and it has some other uses too but these are the main ones. You use estar, for place, feelings, position, and some other. for example yo estoy aqui, i am here. yo estoy enferma, i am sick. el libro esta en la mesa, the book is on the table. The uses of ser and estar are many and it requires time to get to know them and study them all. :)