my boss left for a 30 day vacay

and its like day 2 and this guy who thinks he's the next in charge (he isn't... i think he's a joke) has been like so awful to me so now i have to work 2 times harder and show him im much better than him. on the edge of a burnout btw and ive been to the hospital like 5 times thinking it was a heart attack but whole time it was just a panic attack