For starters, you probably shouldn't have only one team that gets too powerful. Have like 10 Pokemon ~5 levels below the cap instead, and you'll be more prepared for things. You're supposed to think about how to progress instead of just running through it like in Nintendo games. Plan, decide, etc. make it so that you hit the level cap as you fight the next gym leader, then spend the next 10 levels getting to the next gym. It's not supposed to be easy.
Btw, common candies are a godsend.