I can only speak of my own experiences, whether yours in any way relates is for you to determine 
I used to get incredibly frustrated that I wasn’t making progress in many poses. For example, Upavistha Konasana is one of the poses that I was never really able to make progress in. It got to the point that I, early in my yoga practice, took a 35 pound weight, put it outside my door, tied a yoga strap to it, slid the strap under the door so the weight was on the outside, and sat in the pose while pulling as hard as I could on the strap to force myself downward. HAHAHAHAH. Oh man.
There are a wide variety of reasons we don’t progress in some poses. Good teachers will empower us to become compassionate scientists where the subject is our body, mind, and emotions and the test bed is our yoga mats. If trying day in and day out for three months to get your muscles to do something isn’t working, then one might need to take a step back and evaluate if its time to try something new.
“Is another muscle(s) weak or tense and that is causing a lack of range of motion in this muscle?”
“Rather than stretch this muscle, do I need to strengthen it and associated muscles instead?”
“Can I breathe differently as I work this pose?”
“Can I move my body differently, utilizing a different set of muscles in this pose?”
“Do I need to employ other limbs of yoga to help me make progress in my asana practice?”
These and a million other questions might be posed by a yoga practitioner examining their body and practice. And of course speaking with your trusted teacher to get their insight is often valuable as well.
But why might we feel anger, frustration, or other emotions arise in a pose? There are many reasons, a couple which have been true for me are:
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Competitiveness was deeply ingrained in me. I always had to be the best as that would please the people I wanted to be accepted and loved by. As time went by, if someone was better than me at something, those same neural connections would come into play and I’d get angry because, deep down I was scared. Scared of not being accepted and loved.
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Emotion may be associated with the tension in that particular area of the body. Neurologists will tell you, “What fires together, wires together”. So if you had a traumatic event that created that tension in your body, it’s quite common to experience very strong emotions as you begin to release that deep chronic tension.
As I’ve let go of deep seated emotion and come to understand myself on a deeper level, my asana practice has improved infinitely more than just by sitting and trying to crank myself into a pose. Let me also say that I had many teachers tell me I had tight hamstrings. It turns out this was incorrect, it had very little to do with my hamstrings…
How do you let go of it? Everyone will provide a different answer as there are a myriad of ways that are effective. Some might say meditation, some may say asana, some may say pranayama, some might say shaking, some might say prayer, some might say EMDR, and the list goes on. You have to find what works best for you for that particular area of your body. 