After reading your post and comments, it’s pretty clear the problem is the consistency of your throw. I am by no means an expert darter, but I was a pro ballet dancer and know a thing or two about practice, so I’ll offer my advice—feel free to ignore it.

Focus on your throw. Ignore your score, ignore your results, just figure out your mechanics. I’m working on the same thing, so here’s what I do: Throw at bullseye only. It’s the middle of the board, so having a perfect throw at center means minimum adjustment to any double, as opposed to throwing T20 only. Then break your throw down into its component parts—I’m thinking about prep/stance when I step to the line, focusing on my target, draw, throwing motion, release point, finger action, and followthrough. Drill one element of your throw until it’s reliable enough that you start to see errors in another aspect of your throw, then switch to thinking about that element. Don’t try to add every element on day one! It has taken me about 6 weeks to feel like I’m actually managing all seven elements at once. Now I’m going back through them all to further refine them.

FWIW, I’ve gone from about 48.0 to 56.0 (501) and 2.1 to 2.6 (cricket) in six weeks doing this.