It can be overwhelming, so I would recommend planning the scene and the branches in very broad strokes, say, "While swimming to the lighthouse, the selkie MC is beset by a storm and some other obstacles."

Then I would consider exactly what obstacles the selkie will face as they go, and the different ways that they might handle them. Perhaps they can dive to the bottom of the ocean to hide from lightning. Perhaps they can befriend a seabird who can let them hide on its rocky island. Each of these scenes can be as small or large as you like - planning will help them not spread out of control.

As for narrowing the story back, in this example you know that the selkie will arrive at the lighthouse by the end of this sequence - but in what state, and after what experiences, will vary. So in this case you will not have vast amounts of branching to do at the end, but the mood, tone, and details may be different.

It's helpful to keep in mind what scenes are the ones your players will see on every playthrough, as well. There may be lots of these, or fewer, but they will form the foundation of people's play experience.

My human roommate @hpowellsmith likes to code their work before writing it, which helps them keep it (somewhat) tidy. That helps them notice whether the events flow and means they can make sure the code is functional, and correct things more easily than once the writing is in there.

Of course you do not have to do that, but figuring out which way works for you will help you a lot when you start juggling scenes.

What I will add is that an outline does not need to be fully set in stone even when you are coding or writing. But it will be a useful map to help you navigate through complicated paths.

May the stars guide you on your way!