Squeezing chest drawing on harmonica

I would devote the first part of your practice session to fundamentals such as breathing (the most important one!). Then work on applying those fundamentals to playing materials, and observe your use of the fundamental s as you play.

You don't break bad habits. You form new ones and let the old ones lie unused as the weeds grow over them. Part of forming that new habit is deploying it in your regular activities. You can't grow it in a vacuum and then simply introduce it to all your friends.

But do you have a problem? When you say that you're squeezing your chest, are you actually inhibiting the motion of your ribcage and limiting your air intake? If so, then, yes it's worth forming a new habit.

I might recommend that you try the following sequence of actions.

Stand up and stand erect with your head level, like you're scanning the horizon.

Now yawn - really big yawn. Your mouth and throat will open way up, and both your ribcage and your diaphragm will expand to pull in as much air as possible. Do this a few times.

Next, at the peak of the yawn, with your ribcage and abdomen expanded and your lungs fulll, hold that position for a moment and take a snapshot of that state.

Now SLOWLY and gently exhale. As you do, let the ribcage stay expanded while you let your abdomen gently contract as you let the air out of your lungs. Exhale a comfortable amount, then inhale again SLOWLY, doing it from the diaphragm, while you keep your ribcage expanded.

Let the ribcage feel at rest in its open state. Don't be rigid. You're not King Kong beating his chest atop the Empire State Building (we know how that turned out: http://4.bp.blogspot.com/-tbz3K0KAWJM/T0aoELVDgYI/AAAAAAAABYg/XoF6W1PPmhg/s1600/BKliban006.jpg )

Instead, let your ribcage feel like it's riding on top of the tide of your breath, but not moving with it, sort of like a piece of driftwood that the tide has deposited on the beach higher than the subsequent waves can reach.

Now try breathing in and out a few times with long breaths, maybe four second long each. Stay relaxed and breathe easily.

Now try it with a harmonica, playing long chords in Holes 1,2,3, and 4 - a nice, big, oceanic chord. After several repetitions, start pulsing your diaphragm to start each breath, then each beat, periodically returning to a simple long breath. Do this for about two minutes.

After doing chord breathing work, try playing a simple riff that plays one note per beat. Monitor your breathing to ensure that you continue to breathe gently and with an open ribcage. Then try more simple riffs that you can play without much concentration on executing the moves.

Now try working in the materials you've been using in your studies. Take each phrase, lick, or riff, and repeat it several times while focusing on your breathing. When you've worked through enough materials to play one 12-bar chorus, play that chorus while focusing on your breathing.

And on it goes. After awhile you'll find the new habit forming as you consciously apply it and then integrate it into your activities.