Do anything long enough and you’re going to get bored. You’ve got to change–adjust your workout or your workout schedule. Add a new class.
I try to use a variety of different exercises in class so that people don’t get bored. Plus, while it’s sometimes scary, it is also fun to try new things. If you give your body and/or your mind a challenge, it will be easier to stay interested in whatever activity you choose for you health and fitness.