I'm not sure what you mean. Of course you play the piece all the way through. But practicing and playing are two different things.
Here's another suggestion. Do you find that you can always remember and play the beginning of pieces that you learned long ago, even if you can't play the whole piece anymore? Make a lot of "beginnings" throughout the piece. Practice the last page first. Then the second-last page, etc. This way, when you are in performance, as you play through the piece you will be moving toward parts of the piece that you know better and better, instead of knowing the beginning and moving toward parts you know less well. This will give you confidence.
I tell you these things (see my previous post) as an experienced performer who had to learn a lot of things the hard way. I've had poor teachers who gave me advice like, "Play this like a sunset over the water..." and I've had excellent teachers who had had to figure things out for themselves and were able to break it down and explain it clearly.
Topic: How to learn pieces for concerts