I think "diehard," "die hard," and "dieheart" are all different.
noun [ often as modifier ]
a person who strongly opposes change or who continues to support something in spite of opposition: diehard traditionalists | she was a diehard Yankees fan.
ORIGIN mid 19th cent.: from die hard (see die1) .
"Die hard" is a phrase, not a word.
die hard – disappear or change very slowly: old habits die hard.
"Die heart" is nonexistent in a legitimate dictionary, so all we can do is consult definitions that may have been randomly fabricated.
As Kris defined from UrbanDictionary:
One who is ridiculously overwhelmingly passionate about a specific thing, person, place, verb, or adverb. One willing to die for said cause, and as a result their heart would stop beating.
She was a dieheart about the consumption of breakfast cereals. Thus meaning that she valued the consumption of breakfast cereals with great integrity.