Yogi Bear - This is a great question, and important to have clear in your mind.
Straining and working hard are different things. Also, acute pain and an intense stretch are also two distinct things. Straining, forcing, and acute pain can bring risk of injury, while respecting your edge, working hard and breathing through intense stretches can bring immense benefit.
First of all, if you can't breathe you've gone too far. Your breath is your guide in yoga, so keep checking in with how your breath is moving and it can tell you a lot.
Secondly, forcing or straining in yoga is an imbalance of many possible factors. Instead, use just enough effort to maintain full participation in the posture (or meditation, etc) without overdoing it. Soften your face, feel the subtleties of the practice, have fun with it...
Also, and very importantly, acute pain is NOT a good thing, and you should back off, adjust, ask your teacher, or otherwise make it stop. Overall sensation is very normal with a stretch. Shaking happens when you're working hard or as the body releases held energy. But if you feel sharp pain or pain in one specific area rather than dispersed throughout the area being stretched, that means you need to back off and re-align. Something is just not quite right.
Sometimes there is an idea that yoga is all about relaxation, yet if you see some of the fun and wild postures we do, there's no way they can be experienced without a lot of integration and effort. The trick is to balance the effort with spaciousness; to give 100% of what is required but no more. This discernment comes with time and practice, but if you can understand the concept it will help you move toward the experience.
Listen to your body. Sounds like you're doing just that. So glad you asked the questions you did. I hope this clarifies things for you.