Tatyana Ryzhkova gives a nice lesson on how to avoid tension during playing. This comes via her popular YouTube Channel where she has a segment titled Ask Tatyana. I liked the discussion at 12:25 where pushing harder does absolutely nothing to your sound. More of a discussion than a lesson overall, but it’s very important to think about the topic regularly and talk about it to keep it in the forefront of your mind and to constantly engage with the ideas during practice. Some more practical exercises would be nice (although it’s a large topic and tough to fit into a concise timeframe). See more below.
A quick check on your right and left hand technique is also very important. For example, in the left hand, if a student doesn’t play close to the frets with good technique they will squeeze harder to compensate for buzzing or bad finger positions which causes problems even if only on a micro level.
Sometimes the issue is related to stabilization. Here’s a discussion and lesson about right hand stability for classical guitar. The question was regarding a “bouncing right hand” issue. I review some right hand technique and posture ideas and then talk specifically about how to stabilize the right hand.
You might check out this muting exercise that I use with my students as taught here by Gohar Vardanyan. This one is regarding Left Hand Pressure on classical guitar.