I have similar problems. I had to skip the pose â€œcandle/shoulderstandâ€ (Sarvangasana) two years ago after tipping over and straining my neck and shoulder muscles in the fall. The pose â€œwarriorâ€ is very difficult for me, too. Also, the â€œtreeâ€ pose (Vrksasana) with the sole of one foot resting at the inner side of the other legâ€™s knee (or thigh) became too difficult for me (not to mention other, more advanced yoga poses).
Since the substantial deterioration of my balance about two years ago, I have been exercising my balance primarily with simplified variations of the â€œtreeâ€, e. g. not resting the upheld foot against the other knee, but simply lifting it from the ground, thus training simple one-legged-stand. If this is also too difficult, you may try to put one toe or the entire forefoot back onto the floor while resting the heel against the ankle of the leg you are standing on.
I try to exercise standing on one leg several times per day in order to remind my body as often as I can of what I wish it to be able to perform. Over the past two years my balance has definitely improved. Whether this improvement is due to the training or to OMS slowly taking effect (OMS since spring 2013), I canâ€™t say. Probably both. Recently, I started to try one-legged standing with eyes closed in order to train my vestibular system instead of using the compensation of the eyes in controlling my balance. This is incredibly more difficult. I can not even count to â€œthreeâ€, before I have to put the foot back to the ground. As yet, I havenâ€™t seen any improvement in this, but I will continue. I am confident, that one day I will reap a benefit from this exercise.
I also find that the pose â€œbalancing cat / ruddy goose / sunbirdâ€ (Cakravakasana) with one arm and the opposite leg in the air is challenging and therefore good training for my balance â€“ without much risk of hurting myself. One day, when my balance will be much better than today, I will try the â€œcandle/shoulderstandâ€ again, but at the time being this seems to be too risky.
Personally, I think, it is a good strategy to take a step back from oneâ€™s performance limit and cut out those exercises, the failure at which puts you at risk of injury or frustrates you too much, and instead simplify, until you master the simpler exercises so well that you feel fit for increasing the challenge.
Wishing you success in adapting your yoga exercises!