Before sitting for pranayama, you should know how to sit, so that turbulence in the body does not take place. Know exactly the end-middle portion of the tailbone and sit in such a way that it runs perpendicular to the floor. Treat this point as the South Pole, and the centre portion of the head of the spine as the North Pole
Iyengar, BKS. Yoga Wisdom & Practice, p 135
You now have the full repertoire of asanas recommended for sitting. Review all the asanas for seated pranayama and obtain clarity about what each asana is intended to do. As you have experienced, it is not easy to maintain the correct position of the body and achieve effortless flow of breath in the practice.
Several modifications are useful to help you sit correctly.
Images and description of sitting with:
the lower shoulder blades to the front chair seat
block against the wall to the sacrum
blanket rolls around folded legs
sandbags on the thighs
Watch the 40-minute video of Geeta Iyengar explaining the art of breathing when seated and teaching a group of students at the RIMYI (Ramamani Iyengar Memorial Yoga Institute) in Pune, India in 2013. RIMYI is the institute housed in a purpose built building designed by BKS Iyengar in Pune.
Insert Sitting.D3-1.OP2013-1
Practice several times with the recording.
Insert the bucket handle effect of breathing from LoP
Study the diagram from Light on Pranayama. It summarises what Geeta Iyengar tried to convey in her explanation and teaching in the video you have just watched and studied.