What if his ex had died? Not murdered or anything, but car accident, sudden, severe illness (Adam Nimoy’s wife was diagnosed with cancer like a month after they were married and she died within that same year.) or aneurysm or previously un-diagnosed heart condition. (Richard Biggs, who was otherwise perfectly healthy, got out of bed one morning and just dropped dead.) or anaphylactic shock (severe fatal reaction from allergy, commonly food allergies like peanuts or strawberries or shellfish), or she was in the military (and he was not) or some other high risk profession like fire fighter, police.
What if she realized that she was gay and had been living in denial of herself all those years? It’s not really either partner’s fault, but it happens. People try to conform, whether it feels right or not, then reach a point where they can’t deny it any longer.
What if they had been separated by physical distance, and the lack of contact put a strain on their relationship? No affair, just they found their affection waning.
What if she was afraid of commitment? Or felt the relationship was going too fast too soon?
(My ex, Stevie, broke up with me on our anniversary, while out on a date to see her favourite band. There was no affair; she hadn’t met anyone else. But I was very much in love and talking marriage. I’d even asked my dad to get my - late - mom’s engagement ring which she’d left me in her will in preparation to propose. Only to have her tell me we couldn’t see each other again. Maybe she got scared. Maybe she didn’t want to get tied down so young (early 20s), but she could tell I was getting serious about us.) It may also be due to the fact she had a history of abuse, and had some shit to work out first. I’ll never know because she ghosted me shortly after the breakup.) * feel free to use these details in your story if you wish, just change the names.