You might also find that tension is building because you are not preparing the thumb fast enough to substitute for the fifth finger on each rotation of the argeggio. Vice versa, of course, for the LH. You should practice very slowly, of course, and experiment with the RH, for example, keeping the fifth finger on the note until the thumb reaches that same note, then instantly expand the hand to prepare for the next pattern. It's all a horizontal movment, back and forth, with no wrist-twisting, which can also induce tension.
Also, this is Chopin, so don't forget you are singing a song and not impressing listeners with sound and fury. There's a big fat tune here, so sing out, Louise.