I'd like to share this very simple grounding, calming visualisation with you. My Reiki master described it to me several years ago and since then I've used it when I need to calm and centre myself and protect my energy. It only takes a couple of minutes, though can be longer if you want:
Sit down comfortably in a place where you won't be disturbed, close your eyes and take a couple of breaths to relax.
When you're ready imagine roots growing out of your feet and digging into the ground, reaching further and further into the earth, digging through the top layers of the earth then deeper down.
When your roots are deep in the ground see them wrapping around a large, beautiful crystal. It can look however you want, whatever springs to mind. I tend to see black tourmaline, which is protective and grounding, and is solid and reassuring looking. Here, in this place, you are centred and stabilised, rooted into the earth.
Next imagine your roots drawing up energy from the crystal. Gentle energising light flows up from the crystal and into your body, filling you with warm golden light. This energy is protective, healing, loving. Stay here for however long you want to.
When you're ready picture your roots gently withdrawing. You can say something like "Thank you universe for this healing, positive energy" if you're comfortable doing so. Open your eyes and allow yourself a few moments to 'come round' to your surroundings.