The struggle us real. I have the novel plan in my head but it takes so much time and it's so exhausting that it's hard to even start after I've stopped. I've been on hiatus since last year or the year before that. I'm just a noob author but veteran reader. (I'm on Webnovel Qidian) it's just so hard to write the words despite knowing what I want to do.
Try to write even a bit daily, that's what I usually do. Once, I went into hiatus for half a year before picking up my first book again. It's on WebNovel too But, if you feel like you're tired, don't push yourself. Or try by writing notes on a notebook too as you imagine the way to expand your story
When that happens to me, I usually write something else, a short story or a one-shot, and then the motivation comes back. You can also try to write only the moments you like the most in a draft, without worrying about the chapters
Listening to music. Print pictures and post it in your wall (pictures of sceneries, a model of your character, etc). That's what i do in order to be motivated.
It's based on what kind of person you are. For me I try to find really really bad novels with a similar trope or story (bonus if it has a lot of views or a lot of praise) to motivate myself that I can do better.
Sometimes writing in short bursts helps me. an example would be to write for 30 minutes to an hour and then take a break. start back up after some set time, but stick to the plan.
I suffer from a short attention span at first it was fanfictions, yea I can write that better (from my view anyway) but sometimes well not sometimes since I still have yet to finish a novel; I get inspiration for another novel. I have like almost ten yet not one is finished
There was the first set which I stupidly tried to all at the same time and then the latter set which I just drafted after the idea came to me. I want to build experience with my fan fictions first before fully diving into my original work problem is all of the original novels are pretty long so I have a long road ahead of me if I want to go all the way the change it to my way. No disrespect to the authors btw, it was their unique idea to begin with.
I think it will help when you write it out as a plan from start to finish about the storyline, the characters, the setting... in detail, it helps keep the plot holes away and you have no reason to stop and thinking unless you actually lost interested in your own story
Sometimes when I want my specific scene to have a certain feel, I look up to my music playlist. For example, there's an intense action scene, I'll whip out hard rock music (like SOPDMOD's "add ME" song)