I think Rainbow Dash isn’t nearly as confident as she would have others believe, she only acts that way to mask a deeply-rooted fear of failure. For example, in Sonic Rainboom, she expected only the best from herself during practice but when that fell short, she passed the blame from herself onto Fluttershy saying she wasn’t cheering loud enough. By the time the competition came around, her wall of faux-confidence came crumbling down when she couldn’t master the Sonic Rainboom in time (or even perform it).
Another example comes from Fall Weather Friends where she immediately resorted to cheating once her confidence in her abilities were being threatened by Applejack. She then passes the blame (again) over to Applejack saying she should have specified a no-flying rule in the first place.
If you want a pony with true confidence, I’d have to direct you towards Rarity. She does have her anxieties, naturally, but she always manages to work through them and reach her goals. She’s always walking with her head up high with confidence in her abilities as a successful fashion designer, seamstress, and business owner.
Rainbow is great when you need a pony to stick by your side but her crippling fear of failure and her exaggerated self-confidence to hide said fear could use a lot of work.