The first principle of asana / posture is that it should be comfortable and stable. So, always stretch within your flexibility limits / capacity; no need to go beyond your limit. For example, if you are not able to reach your knees in paschamitonasana, practice within your limits. The wonderful thing is that it will still give you the full benefits of the asana as performed by the one who has good flexibility and has performed perfectly. Therefore, always practice asanas within your capacity and never beyond capacity. Focus your mind towards the stretch so that your mind should not wander. Or you can mentally set your counts (reverse like from 10 to 1) while practicing postures.
While forward bending, breathe out and while backward bending, breathe in to facilitate the body have those movements.
It is good that you aim to stretch to the maximum but never fear that you would lose my flexibility and grow fat if you do not do so. Just follow the principle of yoga - sthiram sukham asanam and never deviate from it, then it will give you full benefits.
For more clarity, you can also visit our slide share presentation at
https://www.slideshare.net/GulshanKumar11/basics-of-asana-pranayama-beginners-guide-84363036