Relationships don't end from a lack of love, they end from a lack of attraction.
But here's the deal.
It's not her looks that you find attractive. It's the state she's in. That's right, the mental, physical emotional state she's in, and that you're in too which makes her attracted to you.
The thing that makes relationships passionate is really the polar energies: masculine and feminine.
There are three things you can each do to create the passion and keep the attraction going, therefore the relationship going:
For her, don't let her feel unseen (most women cheat because they feel unseen); don't let her feel not understood (we can't understand, so just empathize); don't let her feel unsafe (give her your presence, this means be calm in your presence too!)
For you, she can't let you feel criticized or coach (many women don't know they're criticizing, they think they're coaching but both are no-no. If she criticizes once she's wrong); she can't close off to you (she will lose her radiance and become masculine); she can't control you (men hate themselves when they're being controlled.
Hope this helps. Let me know if you have any questions.