If there's one thing that has the biggest impact on your success, in whatever arena: fitness, business, personal life - literally anywhere - it has to be your mindset.
It's easy to fall into the trap of believing that you're doing your best, pushing yourself to the limit, when in reality, there's a lot more left in the tank.
Every half-hearted workout that I pushed myself to complete anyway still added up to something, and that something, was better than the nothing that I would have done instead