I'm rated about 1900 USCF and have been stuck between 1800-1900 for a few years not really knowing how to break through to 2000+. I have a good amount of positional understanding and solid foundation in tactics, i.e. I've gotten quite good at solving tactics problems.
That being said, I lose a lot of games due to poor evaluation, time pressure, and erratic blunders. I think that essentially everything in chess goes back to the skill of calculation, including things like time pressure, which is due to inefficient calculation.
So then, how do I further improve my calculation? More visualization exercises? Playing blitz until I can't see straight? Mental exercises? I feel like my visualization is already decent, as I can successfully play 3 blindfolded games at the same time (that's the most I've tried at least). I just don't understand what I've been missing out on that improves my calculation. And if it matters, I'm 20 years old.