Part of it is practice, but part of it is about offloading that mental effort. You can do this with notes etc, but it is also okay to let your players take off some of the load. This is maybe not so much of an OSR mindset, but there's no reason you have to play "pure" OSR. Partially this can just be breaking keyfabe and admitting when you made a mistake, or forgot something, and expecting players to roll with things a bit. But it can also be allowing players to come up with more of the details and plotlines etc. This can be in the sense of players actually building the world a bit, but a more OSR way would be about planning the plots and outcomes less - i.e. you set up the situation, but it's the players who drive the plots.

If players drive the plot, then it doesn't really matter if you forget some essential information, because there's no such thing as essential information anymore. Players won't get stuck when you forgot an essential clue or item because no clues or items are essential. NPCs, rooms, or entire cities and regions don't need to exist if your players never visit them. The plot can't be derailed because the plot is whatever your players are doing and trying to do.

Instead of "there is an evil lich, who is invincible unless you destroy the five tokens he has scattered in deadly dungeons throughout the lands, and you must uncover clues to discover the locations of each token, and how to defeat each dungeon in turn", you say "there is an evil lich, and you can kill it any way that makes sense". Maybe the players train a village into an army - but that's costly and difficult, and still might not work. Maybe the players try to find some arcane sage who can teach them how to counter the lich's magic. Maybe you dropped, for flavour, some rumour about a distant land with fireworks, and the players go on an epic journey to get enough gunpowder to blow up the lich's entire lair.

You can save yourself a lot of work if you keep things as open as you can, and let players drive things. It means that mistakes and forgotten pieces just become part of the lore of the world, and whatever happens as a result of forgetting some detail is just what happens next in the plot.

I think the Kevin Crawford books have good advice on this - it's not fantasy, but Stars Without Number has a free edition that has 90% of the content and has great GM advice on this sort of thing, and it very much agrees with how I've been approaching GMing. I imagine his fantasy book, Worlds Without Number, is very similar.