Somewhere between freewheeling along at a task, and banging your head against a brick wall, is the optimal level of difficulty to tease out maximum learning. The sweet spot of skill is something you need to identify and move to when practising, if you wish to earn sustained improvement.
Mental re-wiring
As we discussed before, sustained improvement of skill or technique is a physical re-wiring of your brain. The intensity of engagement contributes to this (passively practice won't be encoded in skill). Research studies have observed that for this to be achieved, there needs to be a certain amount of struggle present, at the point of learning.
Coasting...
If you get into the groove and begin to flush several 7 irons in a row to the same target, it can begin to feel easy (it's all relative!). However if you carry on like that, and no additional difficulty is added, there's a point where your brain isn't learning anything. It might give some short term confidence but no hard-wiring will be going on.
Vs. Head against a brick wall
Conversely if you get out the 3 iron off a bare lie in a strong left to right wind, whilst also working on your downswing, you might experience the opposite. When you start failing too much, the experience is so demotivating, and the negative feedback so great, there may not be much learning going on either.
The sweet spot of skill
The sweet spot of skill (or challenge point) is where the task is pitched perfectly, just beyond your current level of ability. As a general rule, if you have a 2 out of 3 success rate, you're about there.
Emotionally it is engaging (and thus rewarded neurologically) because although success is just within your reach, it demands your full attention
The role of task progression
Task progression is what coaches do in a lesson to adjust the difficulty of the task. Example: If working on irons low point control, I might give a client a fluffy lie to begin with (or off a tee), progressing through fairway length to a bare lie. An elite player might even get a ball in an old divot if I haven't had my morning coffee.
Other ways of adjusting the task are:
Small swings vs. fuller swings
Slow swings vs. faster swings
Same target vs. changing targets
One club vs. changing clubs
Self coaching the ladder of progression
When practising, it is important that you are mindful of the difficulty of the task and be prepared to make it harder or easier. Imagine it as rungs on a ladder. If you go through a bad section in practice, and your success rate has dropped to 0/3, climb down a rung or two on the ladder. Go for a more lofted club, or if putting go back to 2 foot putts for example. You get the idea.
Technical work
One of the mistakes golfers make is not adjusting the task when working on their technique. In this mode, your attention is more on how your body and the club move, and less on the outcome. So to make the task too difficult at the same time, risks falling between two stools and neither achieving the technical goal, nor the outcome.
In drills and practice games, there is always an easier or harder version. Use your imagination, but make it a habit you seek out that sweet spot - the right amount of struggle.