There s a difference between enthusiasm and being overly ambitious. Enthusiasm would create the feeling of a challenge in your body. I just trusted that the teachers knew what they were doing. I had to learn that they may not be aware of the posture of your body. So always pace yourself and look out for your own needs.
Certification exists in some traditions.
In Iyengar they have a certification process and it takes a lot to become a certified teacher. I am not certified, but I ve taken a lot of yoga classes. Other traditions don t have certification. It s sort of a big thing in the yoga community now because insurance companies are getting interested in paying for yoga. So there is a big national discussion about certification.