Deciding between imparfait and passé composé is a topic that caused me a lot of problems, questions, and confusion when I first studied it. It did so with me and it does so with a lot of other students. What helped me was to try to "feel" the difference and not to attempt to pin down the exact rules. 

It was a big help to me when I realized that the French imparfait had a lot in common with the English past continuous tense. Let's have a look:

I was loving school. -- This implies that, at some time, you stopped loving school. Something along the line: I was loving school as a kid but as a teenager I hated it.
I loved school. -- The simple past tense in English is just a statement of the fact that, at one point in the past, you loved school. You may still love school today. The same is true in French.

J'aimais l'école. -- You once loved school but you don't anymore now.
J'ai aimé l'école. -- You once loved school and you may still love school today.

The other aspect to bear in mind is that some French verbs naturally go with the imparfait. These are verbs that describe a state rather than an action (the most common one is être ) and verbs like vouloir, penser, savoir. Here's more on this: https://www.lawlessfrench.com/grammar/state-of-being-verbs/