Gotta lose that huge backpack until you get more comfortable on the board.
I agree with the stiffness comments. Use your knees like shock absorbers, not peg legs.
My best tips would be 1. focus on bringing your hips forward on your toe side. Like full on hip thrust. This aligns your hips, knees and toes over the toe edge of the board = more control and confidence. Bring the hips back for the heel side, but not a full on sitting position. You want your body position to be symmetrical to your toe side, so just align your hips over your heels for heel side turns.
Once you do that, 2. start leaning your whole body (board included) to start some carved turns. You really need to tilt that board on edge to get the carve going. Imagine there is someone downhill of you and you want to show them the bottom of your board while turning. This will get you making carved turns and really clean up everything you are doing.