Agree with all of above... ^^^
Cement Feet Syndrome - you are probably paranoid about letting the board surface move away from the horizontal position.
Try some simple exercises on the board.... hold your upper body core still, and wobble the board with you legs..... Visualise you upper body and head staying in 1 location, and use your legs to wobble the board side to side. This gives you a feeling for the angles you can tilt the board.... It's similar to standing on a couch, planting your feet in 1 location, but running on the spot on the couch, without lifting your feet off the couch. Try that at home. Then try the motion on the board over water.
Then once you get used to that stability/instability, you will become better at lifting one foot off the board to shuffle around. Then over time, shuffle with become 2nd nature, followed by cross stepping.
Couch surfing is probably good training.... Similar, standing on a bed... running on a bed, without lifting your feet off the bed... picture your upper body staying still, but your feet moving up and down rapidly. You can do this same motion on a SUP board as balance training....
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Failing that, have a go of an Outrigger Canoe, sitting down.

Hard to fall off one of those.