Aikido - physical, mental, and spiritual art
There are so many mental aspects, so many mental "lessons" in aikido.
The principle of "Mushin" (no mind) is an aspect of wu wei ("doing without doing", "achieving without effort" etc.). This means not thinking and ruminating on your actions, but rather let them spring forth from your innermost nature. In combat there is no room for thought -- it is distracting and slows you down. When you are under attack you must react immediately, spontaneously, based on thousands and thousands of hours of training. Trying to analyze the attack, or spending your energy and attention trying to foresee an attack -- or even contemplating the consequences of winning or losing -- takes time, energy, and attention. Your mind should be still like a lake surface in the middle of the night, should be like a mirror, ready to reflect everything without delay and without bias, thus reacting naturally and, especially, correctly. O Sensei described this as the "mountain echo", which reflects exactly what it receives without any distortion.
"Fudoshin" (unmoveable mind) means not being affected by what happens around you, not distracted, not scared, not excited, being able to observe what happens with a totally unperturbed mind.
In the words of Adachi Masahiro:
Real resolve, real fudoshin, is when, in the midst of fire raging all around, you realize there is no way out, and you sit there calmly, as if you were having a smoke of tobacco, considering it a reminder of the imminence of death; abandoning yourself, you compose your mind and face your adversary forgetful of the opponent before you and forgetful of yourself, just leaving it up to whatever happens. This is real resolve.
If you fight willing to die you will survive; if you fight trying to survive you will die. If you think you will never go home again you will; if you hope to make it back you will not.
Mushin is the secret of battle.
(Adachi Masahiro, founder of the Divine warrior school of martial arts in Kyoto)
Another good description of fudoshin in battle is given by Takuan, confidant of Yagyu Munenori, master swordsman of the house of Tokugawa:
The art of the sword consists of never being concerned with victory or defeat, with strength or weakness, of not moving one step forward, nor one step backward, of the enemy not seeing me and my not seeing the enemy. Penetrating to that which is fundamental before the separation of heaven and earth where even yin and yang cannot reach, one instantly attains proficiency in the art.
Letting things happen, flowing with the stream, can be quite important in battle -- and in life.
Achieving the perfect states of "mushin"/"fudoshin" is not easy, but through being aware of the mental aspects of aikido training one can strengthen these possibilities of the mind. I believe this could be beneficial both for achievement and mental health.
There are physical, mental, and spiritual possibilities in aikido.