"Mental limits to push or overcome? advice on how that's done"
I find sometimes that training alone (gym work, cardio etc) is sometimes more difficult then compared to working out with a partner. Its not as if the partner has to give you moral support verbally, it is enough to see that the partner is exerting himself and that you join him doing it. Sort of like a mini competition.
If I'm out of breath and tired, I grit my teeth and keep at it. All the while reminding myself that Osensei said don't train like tofu.