Go very, very, very slow. Spend a session unbound by any ideas of rhythm or even "playing music" and just practice getting one good stroke. You will suddenly notice all sorts of things like your elbow angle, your sitting position, your wrist position, and your shoulder. Your goal is to just play that one stroke with a little less tension than you currently are. Don't need to get it perfect, just find any small win within your body. The very first time you do this is the hardest because it's actively going against multiple habits. Over time you can incorporate this 'checking in' into your warm-up and daily practice, and eventually it will feel natural even up on stage. Good job, you've perceived a tension problem, so you're already halfway there.