It's true that psychically means "mental" or pertaining to the mind. Technically you can use the word the way you did. Take for instance the phrase:
The soldier was wounded both physically and psychically.
This is correct, grammar-wise. However, you won't hear this word outside of the field of psychology and you're increasing your probability of getting slapped in the face by a... psycho.
Let's look at a little background on the word. I used the word psycho just now, you used the word psychically, what's with the psy- prefix? If I had to take the wildest guess, I'd say it's some South Korean pop singer who perhaps got over 2 billion views on a YouTube video of an auto-tuned song that nobody understands in which he embarrasses himself and dances like a horse or something along that line. But that would be totally nonsensical, and I have no idea how I came up with such a ridiculous conjecture.
The psy- prefix is used in Greek for words that relate to the mind. The word psychically actually comes from the word psyche, which comes from a Greek word that literally means "breath." From breath came the word soul (anything that breathes has a soul), and from soul came the idea of the mind/spirit. With this in mind, the definition of the word psyche was formulated to be:
The human soul, spirit, or mind.
Of course, the word psychical therefore has the definition of
relating to psyche as you have seen in the dictionary.
Wow, I'm proving you right! Right? Not really. Sorry to burst your bubbles but let me rejoice: Har har, har har. I burst bubbles professionally now; har, har. Now that I had my laugh, it's time to travel in time... to December 8th, 1871, in England. Professor William Crookes publishes a piece called the "Psychic Force", which describes his encounter of someone we would call today a psychic. The only useful text from his writing is the last paragraph (reproduced below), but you can read more on his Wikipedia page.
"Respecting the cause of the phenomena, the nature of the force to which to avoid periphrasis, I have ventured to give the name of Psychic, and the correlation existing between that and the other forces of nature, it would be wrong to hazard the most vague hypothesis. Indeed, in enquiries connected so intimately with rare physiological and psychological conditions, it is the duty of the enquirer to abstain altogether from framing theories until he has accumulated a sufficient number of facts to form a substantial basis upon which' to reason. In the presence of strange phenomena as yet unexplored and unexplained, following each other in such rapid succession, I confess it is difficult to avoid clothing their record in language of a sensational character. But to be successful an enquiry of this kind must be undertaken by the philosopher without prejudice and without sentiment. Romantic and superstitious ideas should be entirely banished and the steps of his investigation should be guided by intellect as cold and passionless as the instruments he uses. Having once satisfied himself that he is on the track of a new truth, that single object should animate him to pursue it, without regarding whether the facts which occur before his eyes are 'naturally possible or impossible.'"
I found an earlier (?!) piece that labeled criticism of many scientists regarding the "psychic force," but makes sure to note...:
Of course that is possible, and if true, would dispose of the "new force" ;— but if it happened so, Dr Huggins and Mr Crookes cannot be even decently good and acute observers, which their scientific reputation warrants us in believing them to be.
...that Mr. Crookes is a very reputable scientist.
From the Wikipedia page, we know that he probably regretted publishing this because it was all a fraud:
In a series of experiments in London at the house of Crookes in February 1875, the medium Anna Eva Fay managed to fool Crookes into believing she had genuine psychic powers. Fay later confessed to her fraud and revealed the tricks she had used.
But that didn't stop the people from freaking out. From then on, people started believing in the "psychics" the professor described, and movies, books, and other things started featuring those abnormal human beings who have supernatural mental powers.
Jumping back to the 21st century, in, uh, Allentown(??), if you go and ask people what
psychic means, they'll tell you it refers to someone who can read minds, talk to spirits/the dead, etc. This is because, thanks to our culture, the word evolved to become a noun from an adjective that has been used since 1642 to mean "relating to the mind" with Crookes' definition of a psychic. So your 'pals' at reddit know the noun 'psychic', and thought (wrongfully) that psychically means
relating to the noun. The word psychically is only related to the adjective psychic (which means "relating to the mind") and has not evolved to accommodate for the noun we added about 229 years later.
Psychologically can also mean the same thing and you're still grammatically correct. However, psychologically is usually never used because it's pretty ambiguous when you want to just refer to the mind. You'll only see it when people refer to the field of psychology, for instance:
Psychologically speaking, smart people are smarter than dumb people. -Confucius (jk).
The word psychological, on the other hand, is used commonly, as in the statement below:
Son: Mommy! I think I have AIDS.
Mom: It's just psychological. You're too ugly to find a mate to contract AIDS from so don't worry about it, noob.
But that's because "mental" doesn't fit and "mentallic" isn't a real word.
So psychologically - out, mentally wins and makes the statement sound the best.
The verdict (tl;dr)
- Redditors speak before they think.
- Your usage is correct according to the words' definitions.
- In the defense of redditors, what you said would sound weird to any native English speaker.
- "Mentally" is the best word to use in your question because it's specific and, really, just flows. How do I mentally prepare.
- To know which one to use for next time, it's one of those things that really just take practice to master (as you hear different phrases use different words).
- I spent too much time on this answer and I should stay away from this website.