Let reality be reality. Let things flow naturally forward in whatever way they like. – Lao Tzu
During the Hatha Yoga classes, I keep reminding the students to have a practice of self-acceptance rather than a goal oriented practice. Accept what your body can do and be grateful for that. Know its limits and don't judge yourself. This does not mean that you are not progressing in how you do a particular pose. All it means is that you are appreciating the posture for its own beauty and what it brings you each day. Amazing things happen with body and mind when you are self accepting! The body starts opening up. the mind is more calm and overall the practice becomes like meditation in motion!
Frustration due to in ability to do a certain pose is because our ego gets hurt when our expectations/desires are not met. The mind which is responsible for our thoughts and impulses,is naturally inclined to egoism (asmita). Yoga eradicates these conflicting thoughts by controlling the mind, emotions and reason.
The practice of yoga aims to overcome the limitations of the body. With time and disciplined practice, yoga asanas cure the unsteadiness (angamejaytatva) and uneven breathing (shavasa-prashvasa) in the body. Yoga illuminates your life. Regular practice will bring you to look at yourself and your goals in a new light.
So, let go of any resistance and ego while practicing a back bend or balancing in a tree pose.
Accept who you are, breathe and let it be...