In previous versions of the Pokémon games, repeated breeding and catching was the only way to get a Pokémon of a specific nature. That said, the Sword and Shield version of the games saw the introduction of a new feature that allowed trainers to change the nature of their Pokémon without any problems. Fortunately, this feature is also available in Pokémon Brilliant Diamond and Brilliant Pearl.
How to change the nature of a Pokémon
Just like Sword and Shield, Trainers can change the nature of their Pokémon by feeding them Nature Mints. There are mints for every nature and all of them are available in the Batlle Tower shop. Each mint will cost you 50 BP, which is a bit expensive.
Note that you can only access Battle Tower after beating Elite 4 and acquiring the National Dex. Hence, the feature of changing a Pokémon's nature will not be accessible until you reach the end of the game. Once you've purchased a mint, simply gift it to the Pokémon whose nature you wish to change. You can check the nature of your Pokémon by opening the menu and selecting the Pokémon tab. Here, select the Pokémon whose nature you want to determine and then select Check Summary. A new window will appear and the first line will indicate the nature of your Pokémon.