I find the sitting posture too uncomfortable
It is common for most people to experience some physical discomfort when sitting for long periods of time such as minor circulation problems, tingling etc also the urge to relieve an itch or change your posture. This is particularly so when you are new to sitting and it does help stay and to sit with the discomfort and not to give in too quickly. If you find an exercise or posture too uncomfortable or difficult to do for any other reasons, experiment and adapt the exercise and find a way that is easier and most comfortable for you.
If you notice that you are getting tense or anxiously caught up with ideas about the correct way to sit or have questions about whether or not you are breathing properly, remind yourself that you are not trying to achieve anything particular or special, just come back to your breathing. If you need to re adjust your posture and find a more comfortable position, in short, just do it, it is as simple as that.