What does it mean when someone tells you "you're creeping into my guarded heart"?

A guarded heart is held by someone who is worried about getting involved romantically or just emotionally, they have tried to stop others getting in so as (usually) not to get hurt.
If you are creeping in, this does not necessarily mean that you are doing anything wrong but that the persons defences are unable to keep you out as you slowly gain their trust and to some degree affection.