Is always the hardest but that's what differentiates the winners from the losers.
One more rep.
One less beer.
10 more min of running when your legs are already shaking.
You need to find a sweet spot of when training or whatever you do is challenging enough you are pushing over the limit at which most people are going to quit. But not high enough to burn yourself out /overtrain.
Disclaimer: overtraining RARELY happens to us, normal humans who work our every other day for an hour. Unless you're a professional athlete, it is very unlikely that you'll overtrain. New lifters mistake DOMS (delayed onset of muscle soreness, which occurs 48-72 hrs after resistance training) with overtraining. Roll out and eat a banana, it'll go away. You're fine. You get used to this.
More injuries happen due to improper form, or daily activities such as moving furniture, playing with kids or pets, and even heavy groceries.
So, get a coach to check your form, tighten the lifting belt and focus on getting 5-10% more after it feels like your tank is out.
You'll be amazed at the results.