Does erotic literature count as porn in this situation? If not, she could try reading some steamy lesbian smut and see if it does much for her!
If it does count as porn, then perhaps you could find a therapist who specialises in sex or lgbtq+ issues to talk to together - your experience is by no means unusual, despite being every bit as difficult
Other than that, if you folks have sex, then she's been having sex with a girl for ages and enjoying it, so she's almost certainly not straight ;P
In that vein, perhaps just having sex in a way that really drives home the fact that she's having sex with a girl? Discuss with her and ask her to do things like... Touch you in a more typically feminine way, like really caressing your body, your hips, your breasts, your ass, your thighs. Telling you you're pretty etc when she dirty talks, if she dirty talks. If you don't already wear makeup or lingerie when you have sex, you could do so!
Good luck, this is never an easy time, but you'll find your way the best you can