If you are only playing one piano you should say: "Playing the piano helped me relax."

You must always say, "playing the piano" and never "playing pianos". A better way of saying what you want to say is, "Playing the piano relaxes me." Simple, straightforward and clear.

People would understand what you mean, but perhaps one of these phrases would sound better: Playing pianos helps me relax Playing pianos made me relax Playing pianos make me feel relaxed