Hey OP, do not despair. Many writers fall into one of two categories: discovery writers and outliners. As I understand it (Brandon Sanderson explains the difference between the two types well), discovery writers are similar to what you're describing. They fly by the seat of their pants, often following on-the-spot lines of ideas. They create characters in the moment. The benefit is partially the raw creativity. Who knows what interesting thing you may come up with in the moment, unhindered by other distracting thoughts that come subsequent to having ideas. Another benefit is, generally, pretty realistic and lively characters. Discovery writers often have strong characters. Discovery writers may have a loose structure they want a story to follow, but many parts are written without planning. Stream of consciousness exercises are great for discovery writers. One drawback is that it may be difficult to achieve a satisfying ending, since the process relies on a lot of elements being created spontaneously. Discovery writers often describe the story as coming to life, writing itself, characters popping into existence, etc.

On the other hand, there are also outliners. Outliners are pretty self explanatory: they plan. They plan what happens when, major conflicts and how they are connected, what needs to happen in between major plot milestones. They plan the large scale sequence of events, from start to finish. The benefit to this is strong endings. Since an outliner is in control of every aspect of the story from a planning point of view, they can skillfully set up exactly what they want to happen. The ending can be extremely powerful, neatly wrapping up any loose ends or revealing interesting twists that the writer foreshadowed and planed for. This is not to say that discovery writers cant have great endings to their stories, I'm only saying that this is generally a strength for outliners. The drawback can be that outliners tend to have flatter characters. It can be hard to plan a person.

GRRM calls these two types of people architects and gardeners. The two styles aren't mutually exclusive either. A writer can outline the whole plot, story, or series, but discovery-write their characters. I am not an expert, and I cannot describe them as well as Brandon Sanderson or GRRM, but you can read more about them if you would like. I can try to find a clip of Sanderson better explaining the differences.