I am visiting a psychologist.
This implies you're visiting a psychologist right now.
However, if you say
I am visiting a psychologist today - it's totally correct since Present Continuous should be used for future events along with time adverb. Please read more in Collins Dictionary here
Another use case for Present Continuous is:
a temporary activity, even if it is not happening at the time when we
Obviously, you can use I'm visiting a psychologist in this context too. For example, here's a quote from Financial Times:
“I’m seeing a psychologist because I can’t sleep,” she said.
She implies it will take some time before she can sleep again (she will be seeing a doctor during this period of time). Then, she will probably stop seeing a psychologist.
I visit a psychologist.
This implies that you visit a psychologist regularly.
If you say
I visit a psychologist at 3:00 PM today - you plan to visit a psychologist based on your previous arrangement.