I think.. The simple past; estuve is for when you talk about a time in the past, almost like a certain short incident (or a longer period but maybe a long time ago) like or 'I was late because of that' or 'I was in college then' The imperfect; estaba is when you describe being in that situation at the time, and what you did, so it becomes kind of background information, like 'I was.. at the time'

When thinking about the difference between estuve and estaba, it basically comes down to the difference between the preterite and the imperfect. The preterite is for actions that have finished. The imperfect is for actions that do not have a clearly defined completion. The preterite is used for actions in the past that are no longer happening. The imperfect is used for describing states of being or action in the past. Sometimes a context requires a specific tense, but sometimes they can both be correct, but it will change the meaning. Examples: Estuve en mi coche por 23 minutos. I was in my car for 23 minutes. This implies you were in you car for 23 minutes and that you no longer are. Ayer estaba en mi coche. Yesterday I was in my car. This implies that you were in your car at some point yesterday, but with no other implication. It is quite strange to say (I believe this is completely wrong) because it is so vague and implies there would be more to the statement. It would be similar to saying "While I was in my car yesterday." The sentence just ends, but the listener expects that there is more to the statement than simply the fact that you were in the state of being inside your car. Ayer estaba en mi coche cuando me llamó Yesterday I was in my car when he called me. Now we have a natural sentence. The vagueness of the imperfect serves to describe our situation, and now we have the extra piece of information that we would expect. So if we wanted to simply state the fact that we were in our car yesterday we would use the preterite. Ayer estuve en mi coche. Yesterday I was in my car. The more examples you see, the more natural this distinction becomes. Best of Luck