try what is in https://www.accesstoinsight.org/tipitaka/mn/mn.020.soma.html
you can also think about the past, future and the currently changing phenomena. Experience changes as soon as it arises, like a monkey swinging from one branch to another, grabbing on to next one before letting go, so the expression of conditions coming into play is preceded by what has been and not of what is or will be.
Therefore the line of thinking; 'I am [feeling] bad because of this circumstance having to do with having sat for a long time is going to be changed eventually because the development i described counters both the perception of 'i am', establishes perception of inconstancy and changes how we think about time and how we think and what we know about that which is 'thinking'.
think about the thinking itself and of that which you think about; feelings, thoughts & perceptions.
This could calm it and give a new perspective on things.