Make sure you’re doing proper sweep strokes. Just doing extra paddle strokes on one side in that won’t correct it once it’s started veering off to one side.
Without seeing you it’s difficult to give hints but make sure you’re foot pegs are adjusted so you can push your feet against them, aim at something in the distance (don’t look at the front of the kayak- it’s the same as looking at the front wheel of your bike), paddle into the water about where your ankles are and out as your hips go past the paddle, paddle should be vertical so your top hand needs to be in line with the opposite shoulder(ish) and rotate your torso as you go through the stroke pushing the paddle away from you with your top hand. Make sure you’re not bending your lower elbow too much (you can try paddling with both elbows locked completely straight as an exercise)
Pretty sure there’s a club at cuckmere – try going out with them and get some pointers from their coaches.