The muscle you need isn't what you think it is.
I first learned this lesson as a runner. When you're exhausted and you think you can't run any more, the muscles that carry you through are not in the legs. It's your upper body - arms and shoulders - that will keep you moving. When I started doing yoga, I thought that arm strength was needed for all sorts of poses, from plank to arm balances, when the key muscles are in fact in the core. Inversions? Yep, there are those abdominals again. Having trouble balancing? Try some ujjayi pranayama. Don't even get me started on mula bandha.
What about the balance between effort and relaxation? Sometimes the key to reaching further or doing something you have never done before is not to try harder but to let go. To stop pushing so hard, or maybe not to care so much. To find somewhere in the body or mind where you can surrender.
How do we know if we are doing the right thing to get where we want to be? I'm not sure there's any single right answer to this question. Remain curious. See what arises. Don't assume that you know what is needed. Be willing to open in any direction. Be willing to be surprised.