One of a number of tried and tested technique for teaching work that students find hard to learn found in teaching tricks and tips
plan
Our growing evidence from timely practice is that it is better to ensure that at least one layer of e.g. area is mastered before teaching any layers of perimeter. However often we are too late - the learner has learned them both before and is already confused. The strategy is the same, just teach one to mastery - you may will probably need to remove the other from the learners timely practice to do this. Once the layer is mastered you may move on to teach the other topic or you may prefer to teach a couple of layers from the first topic before teaching the second topic. However if learners know a lot, such as how to calculate the area and circumference of a circle - you may want to use the "look at the units on the answer line" to avoid loosing these skills. The jury is out on this point.
accept
Accepting that learning of some topics will interfere with other topics is the first step to recovery.
...