How do I know when to use j'ai été rather than j'étais...it's all very confusing

Bonjour Susan,
This is a very good question and I thank you for it. I struggle with this myself. Here is the lesson from this site that addresses it:
https://kwiziq.learnfrenchwithalexa.com/my-languages/french/view/5013

The one way that I remember the difference is this:
1) If I am speaking, I typically use the passé composé. However, if I am writing a paper or paragraph, then both come into play.
2) Writing descriptive phrases, I tend toward l'imparfait but for one time historical events, I use the passé composé.

Having said that, there are very clear-cut grammar rules that address the use of each which should be in the link above for the lesson.
Here is another excellent reference site from the UT Austin French department that addresses the difference:
https://www.laits.utexas.edu/tex/pr/tap8.html
And if neither of these links are satisfactory there are others, simply google search
«passé composé vs imparfait» without the quotation marks.
Bonne chance et bonne continuation dans vos études de français
Ron