Based on my understanding (others may have better distinctions between the terms):
ardor means "heat, flame, brightness; heat of passion; loved one"
ignis means "fire, torch, brightness, glow, splendor; rage, fury, love, passion; sweetheart"
flamma means "flame, fire, blaze, torch, fever, glow, passion, sweetheart, danger, flare-up"
incendium means "fire, heat, conflagration"
Ignis is fire in the abstract (and by extension bright things like stars); flamma is a particular fire, incendium a large one; ardor is heat, and by extension fire. Almost any of them can be used to mean "passion," but ardor is the most obvious (and all the more so if you're an English-speaker). So if you want to emphasize the figurative element, I'd go with ardor interior. If you want to be more literal (which can be a valuable poetic device in itself to draw out a metaphor), ignis interior works fine, unless you want to focus more on a specific flame/blaze rather than fire in the abstract.