Why do I feel so bad in the morning after taking 300 mg of Effexor XR daily?

Hi, Beach_ bum! These are some of the most common side effects when starting an antidepressant and generally resolve within just a few days to a week or so... and they lessen each day.
How long have you been taking your Effexor? If it's been more than two weeks you should talk to your doctor as you may need a dosage adjustment. 300mg/day is a very high dose usually reserved for very severe cases.
Best regards, Wildcat

Ditto what Wildcatvet said. 300 mg is an extremely high dose. How is your blood pressure? At this high of a dose, Effexor can cause a significant increase in your blood pressure which might be why you are feeling bad. It can also cause major stomach problems, even at the low doses, so I suspect you are having issues with that too. You might want to ask your doctor about lowering your dose and doing an add-on like Wellbutrin, Abilify, or Buspar. That should help with the side effects and be more effective than the 300 mg dose. I seem to recall a study about the maximum effective dose being 225 mg with no significant benefit of going above that dosage. Just food for thought. Will see if I can find the study details.