Train smarter not harder.
This is often repeated by those that don't know what truly hard training feels like.
If your goal is to be the best you can at what you do there has to be sessions where you feel like you are dancing with the devil. Those sets where you have to punch that bastard in the throat to make it through.
Should every session be that way? Not at all. Rest is a training variable not laziness.
Should you seek to be smarter about your training? Absolutely, but the smarter you are the harder and more calculating you can be with your training variables.
There are many occasions that pushing harder is the answer.
There are also many occasions where backing off is the answer.
In the beginning and intermediate stages training hard will be a good go to most of the time.
Our mental strength makes progress when we indulge in those moments of beautiful suffering.