Mantras can be helpful if it is something that is simple and resonates with you. Something that gives the mind a direction and is a reflection of what your choice is.
I live from my beingness.
I am open to the possibilities of life.
I am love.
They are aids as a starting point for meditation and are also useful before sleep as a direction for the subconscious to explore. Or they can be used throughout the day as a reminder to connect with where you want to be. Remember that mantras or affirmations only work when they are a part of the direction you are already choosing. For instance, using the affirmation
I am a billionaire.
will only create more conflict and disharmony if you still have not examined your (many and hidden) belief systems and psychologies surrounding money and self-worth and are choosing to realize yourself in a new way. It will do that as at some point your mind will observe (and judge) the stark contrast between your current state and you desire and not see any real bridge between the two.
Feel what the words mean. Feel how you are creating your life. Moment by moment. Brick by brick. With every choice. Mantras are a reflection of what you are creating your life to be.