I think confidence comes from knowing you can do well. With some things it is easy. You can pass an exam or get a high score. For some it is hard. Like it is hard to see if you are a good lover based on being with one person.
In the backrub example, it could be that your husband's comments were because he thought his gf needed more compliments because it was new or because he felt she needed to hear it more. When one of my kids cooks diner, I usually compliment them more than I would my wife even though she cooks better. So I hope she doesn't feel the kids cook better than she does.
So be careful in evaluating yourself based on stuff like that. If you are unsure, you can ask your husband later if his gf rubbed differently then you. Maybe see if she had a technique.
I think part of confidence is also being willing to learn more or improve. Or knowing that you can improve.