If you are having trouble outlining, or trouble following or completing a story from an outline that you wrote, you should try discovery writing.
I am a discovery writer, and I finish books. In This Answer I give an overview of starting a discovery story.
I do not outline. However, I know the "beats" of the Three Act Structure. In this other long answer I go through the steps of becoming a discovery writer.
The main idea here is to learn the structure of a story and how it works, and then as you write keep in mind where you are and what the next beat (or turning point, or significant event) is, and about how many pages you have to get there.
The first half of the first Act (about 1/8th of the story) is all about your MC (Main Character, or Main Crew if you have a group), their personalities and their normal world (the setting). In a 300 page novel, that would be about 37 pages. In an illustrated format I don't know, but just divide the typical published length, however you measure it, by 8.
Act I is about 25% of the story (all these % are within about 5%). At the midpoint you will have some "inciting incident", something happens that the MC must deal with, but it isn't the normal stuff they deal with in a day. The inciting incident grows into something bigger, and in some way forces the MC out of their "normal world". That is the end of ACT I. This may be physical (they have to go on a journey) or metaphorical (they stay where they are but there is a new danger, or opportunity, or problem to deal with they don't know how to solve).
Read the links for more detail, it is too long to include here.
But the point is, even though you know the beats of what kind of thing should come next or how long you have to complete the current beat, you are not following an outline. You discover the story as you go. You invent the next beat as you go, and it can be based on what you have already done.
Stephen King is a discovery writer, and very successful. His book is "On Writing" and also details how to discovery write. As he says, all stories will come out somewhere, if your characters feel real to you and make their own decisions, then just like in real life, there will be consequences and sooner or later a resolution will present itself. The only thing you, as an author, must do, is ensure your MC cannot just give up and walk away.
In general I keep an idea of at least one viable ending in mind, even if I don't plan how to get there. But when I am writing, my ending usually changes three or four times; I think of a better ending that fits, or while writing, I write something that precludes the ending I had in mind. When that happens, I force myself to come up with a better ending that fits, or I have to reverse what I wrote so the previous ending I had in mind will work again. I usually come up with a better ending.
But I don't write the ending, I keep some notes on it, things I need to be sure and cover when I get to ACT III, the loose ends and questions I should answer for the reader before the book ends.
So it isn't plotting, or outlining. To me it is more like following a compass setting for a certain amount of time. I know the direction the story has to take, and how long I have to get there, and I know before I start who my MC is and what her big problem will be. So to start, my "compass direction" is to describe her, her navigating her normal world and setting, and I have to come up with introducing the inciting incident.
That's it, and I have imagined my character for a week or more when I start, so that is actually fairly easy to do, and writing about her in her normal world dealing with common every day problems helps make her personality more concrete, so we get to the inciting incident without a problem. For the next 1/8th of the book, I need to gradually escalate THAT problem into forcing her out of her comfort zone (the normal world). I think of ways to do that, and in a month I have a quarter of a book, and a clear direction: My MC just gave up on easy fixes and is leaving her comfort zone to deal with her new problem.
The "next thing" to accomplish is seldom more than a handful of scenes away.
Fair warning, this can involve significant rewrites. If you are illustrating as you go as well, do quick sketches or something fast, just to help you remember what you wanted to draw, something you are willing to ditch if you have to rewrite. Discovery writing is finding a path through a forest, and it is easy to take a wrong turn and end up at a dead end, or end up having circled back to a previous position and accomplished nothing. You really will have to kill some darlings.