Counting reps, when it comes to higher reps like 10 and up, can become a mind game. Especially when doing heavier compound exercises like squats, DL's or benches.
Rather than counting in the regular way of one, two, three, etc, all the way to ten (or whatever the higher targeted number is) might consider this little mind trick. First rep is counted one. Rather than counting two as the second rep you say "and". The next rep after that will be the number two and the rep after that is another "and".
Example: Ten reps the goal..... 1-and-2-and-3-and-4-and-5-and (the last and tenth rep of that set). The brain does not compute the word "and" as a number so it believes that only 5 reps were done rather than the harder 10. Had worked very well for me when I was doing 20 reps heavy breathing squats. I consider lifting being at least 50% mental anyway. Any trick in the book to help lift better and stronger. Good Luck.
Your last rep is not always going to end with "and", it may be an actual number like 6,8, 10, etc. For the last rep, if ending with "and", some guy's will say 'done', 'that's it' or' F**K it'. Good Luck.