The most annoying thing when you switch from online playing to OTB games is that you don't have a full vision of the board.
Getting used to play 2D, you probably have developed your calculating skills with an "above vision". With this kind of vision a player can focus on lines and spots, and pieces do not interfere since they are "flat". But when you play OTB, you have to look at the board only from you side, with pieces messing around. Thus, you miss some threats that you probably would notice on a screen. And usually this is due to the fact that "you don't see the lines". In my experience, I noticed that this happens especially with Bishops and Knights.
You can't "flatten" real pieces, of course. And you have to get used to 3D pieces in order to "see lines" on a real board.
In the meantime, you can try to play chess looking the board "from above": standing near the table instead of sitting, for example.
This will diminish (a little) the traumatic switch from 2D to OTB.