One way to think about it is that we use AVERE when we have an object or a thing with the subject (we are the subject) when doing the action. For example ; ho mangiato! (I have eaten) eaten what? the food! The food is the object that is with the subject! Whereas, we use ESSERE when the subject (me,you..etc) is doing the action and there is no other thing involved. For example ; sono andato! (i have gone /I've gone). As you can see, you've just left somewhere, there is no other thing involved except you as the subject left!

Stephanie, I would suggest starting a personalized list of verbs organized by 'avere'/'essere' which you can compile as you go, based on the verbs you've needed. Then create an "anchor" phrase or two using each verb, and let that anchor phrase be something true and relevant. For example, 'Ho imparato molto oggi', and, 'Sono andato a Roma una volta'. This way it's contextualized, and ready to use in conversation.

Unfortunatly italian is full of exceptions. You just have to learn them by heart as we have to learn english irregular verbs

I'm afraid you'll have to learn them off by heart, but I suggest reading a lot and watching a lot of movies or tv shows in italian, that way you'll get used to the exceptions and they'll come naturally to you with less effort :)

You have to just learn them but this helped me. type into google 'the house of essere'