Nightfall is normal. You are not suffering from any disease or having any psychological problem if you experience nightfall. Anxiety is caused when we don't have correct scientific understanding about something. One can feel weak due to several other reasons also which may or may not be connected to this.
A psychologist is not authorized to give medications. However, therapy does help in these issues.
Next Steps
You could consult in person to understand the problem better.
Health Tips
Please be aware of the difference between psychological therapy n psychiatric medications. From the information provided by you, you don't seem to be needing meds but therapy.