I suppose your question is, why is it not "in mein Herz" or "in meinen Herz", etc.? The reason why "mein" turns into "meinem" and why "Herz" turns into "Herzen" is because it's dative. "In" is a preposition that can use accusative or dative; it uses accusative for movement and dative for location. When you're saying that something is in your heart, then that's a location, hence you need to use dative. The dative form of "mein" for "das-words" is "meinem", and "Herz" gets an extra "en" because it's treated like a weak noun (even though it's considered an exception), and so-called weak nouns get an extra "n" or "en" in accusative, dative, and genitive.
