When someone is stuck in the corner like that don't just treat them as a fucking heavy bag. Give enough space between you and them some you can create leverage on your shots, jab at them till they give you an opening. Split his guard with an uppercut, double up on one side. If he was decent at all he could have timed you easily and countered with a left hook right on your chin.

But man you need to work on the basics, fuck going southpaw, learn how to fight orthodox first. Get the footwork and straight punches down. Your jab had no snap to it. Keep you elbows in and twist your torso into the punch.