Online you should play shorter sessions and less tables. Helps for sure to focus more what happens at the tables. I also find it very useful to make notes.
When you notice that your concentration is slipping and you are making more and more mistakes, stop.
Of course, this only applies to cash games.
If you run deep in a tourney and getting tired, nothing you can do.
Years ago I tried to play 20 tables 6 max at PS. I played horrible. No time for taking notes or paying attention to what happens at the tables. I reduced it to 6 tables and it worked great.
Of course there were guys who could play 24 tables profitably like Nanonoko or Dustin Schmitt. I think those were rather exceptions.
I found "boku" to be the most extreme, playing 50 or more sng's at once.
If you can play well online, you can play well live, but remember that limits are not immediately transferable. A 50 live tournament at your local casino is likely to be quite soft, whereas a 50 online MTT is likely full of very capable players.
It used to be. Trying to figure it all out and make it work without losing your bankroll along w/your mind... Yeah. It can get overwhelming, if you let it. I've found that the acceptance of the game, in all of its surly glory, makes it easier and more fun to play poker. What I mean is that I'm not going to win every time; I'm going to get beaten on the river once in a while; I'm going to suffer the set-over-set syndrome at times; Even the runner-runner will rear its ugly head at me and take all my chips away. Sigh... But I will double-up my stack. I will flop quads. I will take down 4 sng's in a row. I will make multiple final tables in one day. I say this because I have and will again.
The mental pangs eventually vanish, and a sense of automation takes over after time. It's all just second nature, now. My only mental strains now come from the pinwheel that pops up on my screen.
So funny, I actually feel the opposite. I find online poker very relaxing even therapeutic. I'm in my own space and often have a poker stream on in the background. While playing live is very exciting, I find it draining. All the stimulation in combination with the slower (than online) game makes me feel a little low energy.
Is hard to fully focus when you play for many hours. Sometimes is much more frustating than live poker. I would recommend you to have a little distraction while playing, like watch some movie, listen to music or other stuffs. I know you posted this topic on Janury, i hope by now you're better, but the tip stays