In general, it just makes sense to build up your work capacities and stamina gradually. What else can you do after suffering an MTBI? Your brain won't let you go from zero to 60, so you have to go from 5 to 10 to 15 (or from 1 to 2 to 3), etc. and work your way up. Some might see this as depressing, but what other option is there? Who knows how far you will get eventually? I do think that (to varying extents) the brain can be retrained and rehabilitated over time, with patient effort.