Calculation skills to 2000 and beyond 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.

Thanks!
If you can play three games blind-folded, it is unlikely that your problem is calculation. I dare say that there aren't many 1900 USCF players that can do that. 

You say you "lose a lot of games due to poor evaluation, time pressure, and erratic blunders". That also doesn't cry out "calculation problems" to me. 

In my experience ratings first and foremost reflect positional understanding. Calculation skills make a much smaller difference, at least above a certain threshold.

To be clear, positional understanding isn't knowledge as much as a good feeling for the position. I.e. whether you can precisely estimate how good your position really is and quickly come up with sensible moves and plans.
If you are lacking in that department you tend to calculate a lot without achieving much. That results in time pressure and blunders just as surely as having calculation deficits. 

So I would rather recommend a book like Positional Play by Jacob Aagaard. If you can easily solve the problems in this book, you can always go back to pure calculation. 

By the way, there is a big difference between training calculation and training tactics. Solving tactics depends on seeing motives, training calculation depends on the ability to think far ahead without confusing yourself. So if you do go back to pure calculation, maybe you should try the Calculation book by Aagaard, or similar books by Dvoretsky.