Maybe just play shorter sessions?
Think of it like multi-tabling; you start by adding one, play like that for a while, then try 3 tables for a while, ect. You might need to build up endurance in the same manner.
Your concentration might be just tailing off after a certain number of hands, so when you start to feel that, just end the session. There's no point playing when you're not on your best game. Over time, that point might start to come later and later.
There's other possibilities, like a checklist which includes position/cards/reads or stats, draws on the board that you could have printed out, and after a certain amount of time you could try forcing yourself to go through the checklist. For me, I don't know if that would work but everyone's brain system works differently.