In this case who/whom refers to the subject of the sentence, so use 'who': "Be proud of who you are." If the who/whom referred to the object, you would use 'whom'; "Be proud of being a person for whom honesty is important." To distinguish between subject and object, we usually use the structure "Subject Verb Object" in English, so with the statement "Alan likes Joao.", Alan is the subject and Joao is the object. If you didn't know some information you could ask either "Who likes Joao?" or "Alan likes whom?" or even "Who likes whom?".

"Be proud of who you are" is correct. This is because 'who' is the nominative or subjective case; when you use the verb is (are in this case), you are saying that the two nouns are the same thing; thus, they can both be considered the subject of the sentence. Whom is the objective case of the word. You usually use it after a preposition like "to" or "from" or "for"- for example, "You gave the cake to whom?" "You baked the cake for whom?" "You got a cake from whom?" Whom can also be a direct object - for example, "Whom did you see at the store?" I hope that helps!

"of" precedes an entire clause here: "Who I am." of + "who I am" Prepositions can do this, remember? I doesn't just precede the word "who".