I can totally relate about your b/f using his former girlfriend's birthday. I went bezerk when I realized he had used their anniversary date as a code (he was no longer with her) but It's amazing how jealous and possessive we can become. It all has to do with the dopamine high we get if we are completely in love. It's like an addiction so don't hard on yourself. I made my ex-boyfriend change his code; call me controlling but hurt to know he was still using their anniversary date as his code. I can guarantee you they would be more hurt if it were reversed.
Maureen