Psychic is one of the most powerful psychic-type moves you can get in any Pokémon game. With a power of 90 and 100 accuracy, it is difficult to reach the top. This TM is hidden outside of Celestic Town, but you won't be able to get it until quite late in the game. Here's where you can find this TM in Pokémon Brilliant Diamond and Brilliant Pearl.
Before you can get this TM, you must first get the TM100 Rock Climb. This hidden move can be recovered when you are making your way to Snowpoint City and Lake Acuity. Keep left as you cross the blizzard section to find it easier. To use Rock Climb, you must defeat the Gym Leader in Snowpoint City.
Once all of this is done, head to Celestic Town. After you arrive, go out of town to the left. You will see a staircase next to the Route 211 sign. Follow the path up the stairs and fight the trainer near the rocks that you can use Rock Smash on if you haven't already. After the fight, destroy the rock behind them and use Rock Climb on the rock face. At the top you will find the TM you are looking for.