Telling a girl she can be and do anything isn’t just untrue; it’s harmful to her. It keeps her from practicing two vital life skills: recognizing that she is not in complete control of the way her life turns out, and accepting that there are some answers she will not be able to immediately know.
If you’re not 100% sure a situation will go her way, avoid making promises you can’t keep. Sure, it may calm her down in the short term. But if she believes she can control outcomes, she’ll acquire both a grandiose (and false) sense of her own power, as well as a tendency to beat herself up mercilessly when things go wrong. She needs to hear you say, “I don’t know the answer,” and watch you be peaceful in that unknown.