I would use the internal/external duality as well, and treat 'interpersonal' and 'environmental' sources as 'external' and I do think you need both.
If I may chip in, I've always liked John Truby's (The Anatomy of Story) method when it comes to creating an organic plot that is character driven and whose elements, no matter how far removed from one another, all form a web-like structure woven around the central character(s).
If memory serves, the way he addresses internal/external conflicts is this:
- A good character will be defined amongst other things (strengths, weaknesses...) by their need and (not "or") their desire
- Their desire will be the stated main objective of this character. It is an external conflict, or a conflict linked to an external element: An item, winning a war, defeating X, surviving a dire situation, escaping jail, etc.
- Their need creates an unstated objective that will be attained or not as the character pursues their desire. It is an internal conflict, something often arising from a character's past faults or flaws and which they might not even be aware of.
The need and the desire cannot be the same, however they can both find resolution through the same action. For example, in Groundhog Day, Bill Murray's desire is to escape the infernal time loop he has found himself trapped in. But his (true) need is to learn to love life and stop being a grade-A a**hole. And the only thing that will bring an end to the time loop is when Bill Murray starts to love life and stops behaving like a grade-A a**hole.
So there are four kinds of ending:
- The desire and the need of the character remain unfulfilled. A super depressing ending, not only has the character failed but they remain unaware of the nature of their true need or have made it even worse by the end of the story.
- The desire is fulfilled but not the need. A tragedy. The character has apparently won but their true need remains the same or have worsened in the process of attaining this objective. The Godfather.
- The desire is unfulfilled but the need is. Quite a happy ending. Even if the main objective is failed, the character realised that this objective was not what they really needed to transcend their condition, and they have evolved positively in the process. Rocky doesn't win the boxing match (apparent defeat) but he has vanquished his inner demons and therefore emerges as the true victor.
- Both the desire and the need are fulfilled. Upbeat ending, fairy tale. They all lived happily ever after.