I think for you to talk to her about that is a good thing. Showing a bit of uncertainty may not be a bad thing depending on how you express it. If you show it by checking on how she is feeling by whats going on then that should help reassure her that you care about her welfare at all times. That also will help you become more confident as you learn by her feedback whats ok.

I think that confidence is important but I think its gained by experience. You will find that it will grow as you both learn what you like, limits and discuss what worked for you both and what you want to try.