How far off the mark are you? The reason I ask is that I have had the misfortune (and still do at times) of putting up with people who have struggled through line training and never properly made it out of the end, despite passing. The best that these people can manage is career First Officer and the best thing that can be said about them is that take the focus of the check and training people off the people who are marginally better. With one guy, I am convinced the fire axe is installed on my side so I can take out the annoying little knob if ever there is a real problem.
If you are one or two flights out, then this is "just one of those things" but if you are 100% out, like you are at 80 flights and not ready for a line check, look for another job. Really. Flying is a job for dummies and maybe you are too clever. Or maybe you haven't learnt to leave your mistakes behind. Your ambition should initially be to be totally average. For that you can read standard. No heroics. To get there easily you have to be ahead of the job in hand.
All of the posters above have given you "Top Tips." May I add some more? Firstly, don't take any foul-ups into the next phase of a flight. Next, think about what is going to happen next. Going into the Crew Room, you'll be briefing. After briefing you be going to the plane. And so on. Where you don't want is to be is still at cruise level 20 miles after TOD without a good reason which will have been pre-briefed (and considered) 20 miles before TOD, like six minutes perviously!
Chill and take it easy.