I repeat "Om Namah Shivaya" - but that is a personal choice. Traditionally, it is accepted to be a powerful healing mantra beneficial for all physical and mental ailments and it has been defined as meaning "I honor the divinity within me". (Taken from the Wikipedia Page)

But there are lots of mantras out there...

"Shanti Shanti Shanti" - means peace, rest, calmness, tranquility, or bliss x3

And before I started chanting Hindu/Sanskrit mantras (long before the concussion) I would say "Everything is in Divine Order" over and over again to remind myself that it was - especially when I felt like life wasn't in Divine Order.

You can make up your own mantra (or chant or prayer) according to your own spiritual beliefs, or you can find one online, or you can get one from a spiritual leader in your life.

I believe every religion or spiritual practice can include mantras, or chants. You might even repeat a prayer, or recite a hymn, or a portion of a prayer or a hymn that brings you a feeling of peace or solace, or one that happens to resonate with you at the time.