Here are some thoughts. Pick an idea you dearly want to write about, but also one that’s simple and straightforward. Fantasy and sci-fi and world-building are tough to finish for anybody. For NaNoWriMo and for your first finished novel, they’re very tough. You said you like thrillers. So do a cool thriller.
Set writing goals. You know how to do that. But work on making the ending very cool. Something you can’t wait to get to. A neat twist that you have to write. Perhaps this will help you finish.
You have to finish a novel. You absolutely can. Finishing will help you get over the mindset that you should stay in short stories. Short stories are cool and are definitely an artform but nobody buys those anymore. Write a novel. Take a short story idea you have and flesh it out a bit. Don’t work too much, however, on writing the story overview and all those others things. Instead, write! Write and finish a crappy first draft for a novel. I’ve done that many, many times. It feels good.
You have 30 days. 1,667 words a day. 12,500 words a week. It’s easy. Write 5,000 good words a week and 7,500 crappy words. The point is to finish. Right? Then you can figure out how to make something out of the mess.
Pick a fun and fairly straightfoward idea. THEN DO IT.
It’s fun to finish something. Seriously. My best part of writing is the endings. The climax and the conclusion and the hope and the tied loose ends are all very satisfying to write.
Wishing you the best NanaR2D2 month!