What Happens When You Assume?
A breakup/emotional betrayal scene is a common story element, and can be handled in a variety of ways. I don't like the stock good/bad elements, so I always feel it needs to be nuanced so your characters are more than cartoons. This means the betrayer should feel justified in his actions, and the betrayed should have seen it coming OR they are partly responsible. Your villain is simply being internally consistent, even if he is sure in his betrayal. Your heroine was seeking revenge, and that's rarely a pure motive. She used him to get what she wanted, and blinded herself to the things she didn't want to see.
Any time you have a long-standing relationship that goes sour, you need to understand why. These people loved and/or trusted each other, and now that is all screwed up. How does that get portrayed in a sympathetic light? The key is to understand what has changed in the relationship that causes the stable dynamic to shift.
In this case, the prince is now going to be king, or anticipates it. The heroine is going to achieve her revenge. The relationship has been built on a quest. Each now has the dynamic that was pushing them together changed radically. It's almost predictable that they would break up - the glue that held them together is a goal, not a mutual passion for each other. True love would have each ready to sacrifice even their goals for each other. But both have assumed the other is their for them, and will feel betrayed when they realize the other person is separate of what each wants.
A precipitating difference in goals will appear that wedges the two apart. Both are at fault, but the prince is going to take the fall because he isn't the POV character.
- Here's an example I've written.
The MC is a sort of legionnaire, borderline slave, and her teacher/master has a mental illness that makes him dangerous so he's confined from the public for safety. Together they form a relationship where the MC thinks she's exploiting him to get a degree of freedom (but has developed feeling for him she won't admit to herself), while he thinks they have a close relationship but continues to treat her like a slave in many ways. The MC falls in love with someone else, gains her freedom via plot elements, and gets married. The teacher feels he's been betrayed even though he was being an ass. The MC is betraying him, but feels just about it (even though there was a real relationship). Both separate feeling hurt and betrayed.
- So I'll go with an example for your story. This may be a little too close to telling what to write, but it seems like the obvious choice.
So being a prince is a complicated thing. I hate to have a good, long-term character turned into a total ass. He should genuinely believe he's doing the right thing on some level. So my suggestion is, kill the romance by having him get married to someone else for political reasons.
Your female lead has imagined for years that her love and devotion have secured a place of loyalty from him. But a person raised to be a prince would decouple love and marriage quite easily. "Hey, baby, why aren't you fine being my mistress? Dad had a mistress, and Grandpa. Mom was awfully friendly with her 'ladies in waiting' and it's not like I care for the Duchess Brunhilda. Surely you always knew I'd marry for power?"
Once you've dashed her illusions, she starts re-interpreting all the stuff he's done over the years. If he DOES marry Brunhilda, maybe Brunhilda has her own feelings on the subject and pressures him to conform to 'conventional' morality. The prince's new wife might not be so secure in her relationship with him to be trusting. Brunhilda keeps the two apart, NOT communicating but only sharing a common goal related to revenge. And now that the prince is King, he may have legitimate political reasons not to support a socially disruptive revenge scheme anymore.
So everyone very much feels like THEY are the one being betrayed. It's a difference in class and social status that I've always found frustrating with the whole 'rescued by a prince' trope. Once the prince realized his girlfriend isn't okay with his wife, he starts interpreting what SHE does as disloyalty, lending well to your initial plan if you want to. He's subconsciously distancing himself from her emotionally, and who knows, maybe she really IS considering taking up with his brother (who, if not in line for the throne might actually be a possibility for her). The prince may have even encouraged his brother to start a relationship, but now the prince feels betrayed she actually went for it (feelings don't have to be reasonable).
- You can use any kind of plot element to introduce the conflict (she wants to kill an enemy the prince needs as an ally to gain the throne, etc.) but it will ultimately come down to both assuming the other will sacrifice for their benefit, and neither willing to accept the other person is more important than goals.