Are you working with a teacher? Sometimes we are so busy trying to play, that we are unaware of how tense we are. This is something that a teacher can help you become aware of.
One way to diagnose excessive tension in the fretting hand is to play slow scales using "place then play" as two steps. When you place your finger to play the next note in the sequence put absolutely no pressure on the string. Check that the other fingers are not tense and exerting no pressure on the string, even though they may be resting on the string. Then play the note, pressing only enough for the note to sound. You can practice "buzzing" the notes on the first time through if you think that you might be using excessive force. After playing, release all pressure on the string, playing the note staccato (but don't lift the finger off the string- just release the pressure). After becoming comfortable with staccato, you can try playing more legato, still being aware of releasing the pressure in the other fingers when the are not playing.
This exercise can also work on synchronicity; the right hand finger prepares (places) the finger at exactly the same moment as the left places the finger on the string.
When I first started to do this exercise, my left and right hands did not prepare at the same time and the fingers of my left hand were still pressing even though they weren't playing. This causes extra tension in the hand and that leads to more tension in the forearm.
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Classical Guitar forever!