Tennis looks very simple when you watch a good player. But the road to mastery is long and it doesn't matter which area you're working on.
Remember – the brain stores EVERYTHING, not just stroke mechanics. It stores emotions, thoughts and consequently beliefs TOGETHER with the mechanics!
Beginners are not actually aware of the number of balls in the basket when they enter the court. If they would start their volleyball, soccer or basketball practice, they would see 10-20 balls. But here they might see from 40 to 200 balls or even more in one basket.
When a coach reminds you to play gently and you forget to or decide that it's boring to play gently, then you will very likely overhit. You can correct that mistake by hitting more gently. It is in your control. Unless of course you're late on the shot and you're hurrying the stroke which produces an overhit.
You cannot have a perfect technique after 45 repetitions. It's not possible to do the correct and fluid motion with so little repetitions. Your brain needs many more to store and coordinate your body fluidly. You also cannot have a good feel for the racquet face level or swing speed after 20 minutes of practice.