Pretty much all students do that at some point. So don't feel bad if you recognize yourself here. I was also guilty! When I was in school, my teacher constantly had to remind me to choose one tempo for the whole piece and stick to it. Otherwise, it's not music anymore.
The best is to work on the hard passages and once you've established a reasonable tempo for those parts, play the entire etude at that tempo. It'll feel a bit slow for the easy parts but that's okay. It's better to play an entire piece a bit slower than slowing down and speeding up all the time.
Eventually, you'll become stronger and faster and will be able to play the entire piece at the tempo you want.