that's tough to answer without actually seeing you play.
In a general way, i would say that tension = grabbing, clenching and immobility.
So to counter that state, you have to do the opposite, that is let go loose, and move a lot, i.e. activate the hand, fingers,knuckles, wrists, elbows by moving them in every which way.
At first, chances are you'll be playing all kinds of wrong notes, feel totally stupid, like a rag doll, but out of this you'll discover things. Forget all principles or dogmas you may have as to how you should play or how you should hold the hand.
Experiment with stuff you've never done before and without a coach you have to simply try all combinations of movement you could think of. Within all that jumble of possibilities i'm pretty sure lies your solution.