CrossFit is successful for so many reasons. It builds community; it's fun; it's challenging, it makes you better. When you first begin CrossFit it can seem as though getting through the workout every day is a success in itself, and it is. It's amazing to see our bodies perform in a way we never thought possible.
Then we progress, and all of a sudden running 400 meters is no problem, and it's possible to get through the 20 pull ups that day's WOD requires. As you progress through your experience with CrossFit I would challenge you to challenge yourself.
I have seen it time and time again, when people start to get comfortable with using bands for pull ups, or an empty bar for a workout. Challenge yourself, take that band off the bar and do the pull ups RX'd. Sure, you might finish slower, and sure it might be hard. BUT YOU WILL HAVE GOTTEN BETTER. You will leave the box that day better than when you walked in, you will have done something you didn't think was possible.
Maybe you need to bump your weight up to RX'd, or stop breaking up your sets, whatever you need to do to get better, DO IT. Remember that the amazing thing about CrossFit is that athletes of all different abilities come in and do the workout, that means you get out of it whatever you put into it.
It's up to YOU how you will progress as an athlete.
That feeling is what makes people great not only in CrossFit, but in life. You start to believe that the impossible is possible with every aspect of your life.
So, get in the gym and challenge yourself, then go out in your life and do the same.