Your confidence is only undermined if you think it is undermined -- you require neither skills nor knowledge to feel self assured. Just ignore the advice.
There's nothing wrong with your cadence for the type of riding you do. As far as using your smallest chainring goes, which cog are you using in the rear? If you are using the smallest one (or almost the smallest one), you should be using a different ring in front because cross chaining will increase chain wear and noise while giving you less gear selection. If you're using a cog that's at least a couple down from the end, you're fine as you are.
To get back to your original question on how to deal with her, just speak in declaratives. When she tells you what to do, just say "I like to do X, and it's hard to motivate yourself to ride if you don't have fun, so that doesn't work for me."