Proudly and pridefully are a pair that is commonly mixed up. They're often used interchangeably, and though this is not "wrong" so to speak, the deciding factor between the two should always be the emotion the writer is attempting to evoke or convey.
Proudly
The term proudly should be used when attempting to convey a positive sense of self accomplishment and satisfaction. The term itself carries a positive connotation, though it is versatile and can be used to convey a negative sense of accomplishment as well. You're proud of your children on graduation day, proud of your significant other when you show them off, proud of the promotion you got at work, and proud of the new car or house you purchased. When you show these people/ things off, you do so proudly, which is a positive thing.
Pridefully
The term pridefully carries a very negative connotation. It portrays arrogance or stubborn refusal to humble oneself. Unlike proudly, pridefully only carries the negative connotation and is never a positive term. Think of pridefully as meaning "full of pride," or even being unreasonably overcome with an overbearing and all-consuming sense of pride, an attitude that is never good to possess. If I say, "He pridefully walked away," it means the man the sentence refers to walked away with a sense of arrogance, inflated ego, or unrealistic sense of self-importance.
Though their meanings are similar, these words are not truly interchangeable. While proudly could be used in the place of pridefully, pridefully should not be used in the place of proudly, as it carries a consistent negative connotation.