I write short stories. I’m just practicing. I have been doing this for fifteen months. I still have a long way to go. Now I’m rewriting a story I wrote a year ago. It is/was a boy meets girl thing. She is older than him. Something like 21 and 25. I wanted the age difference, and I used it a little. But as i rewrite I see I can use the age difference much more (he is also 20 now) I can get much more friction and tension. More drama. It makes it a better story. It still ends on high note. They end up together, I didn't change that. But anyone can see that this is not going to last. I have made them too incompatible. Stories can end good or they can end bad. But is it OK to end a story in a happily ever after way when a breakup is around the corner? Of course one way to do it would be to make it twice as long and follow them to the bitter end. Lazy, soft, coward as I am, I actually toned the differences down again. I felt I would be dishonest to any reader leaving a happy ending with a disaster nearby. Thoughts about that?
What makes it a disaster? A four-year age gap isn't huge, even at those ages. If it's some other factor....then if you do want it to be a convincing happy ending, show that they're at least trying to overcome the differences. They don't need to succeed within the scope of the novel, just make it clear that they might succeed, and that should be enough.
Personally, I have found that I prefer bittersweet endings. I think the reason for this is simply that it is more realistic, as well as slightly emotional. Though, I am mainly thinking of films here, in all honesty.
I don't see anything wrong with the premise outlined in your post. But I don't agree with the title of the thread that "love conquers all". Sometimes it doesn't matter how much you love someone. If it won't work, it won't work. It's up to you how you play that out in your story though
Imo if you weren't explicit that their relationship is doomed then it's up to your audience to decide. And if you did make it clear they won't make it, then I can't imagine the story ending on a high note.
The story should tell you how it should end. I've written two books with romantic stories. One ends 'happily ever after'. The story is almost completely about two people conquering obstacles to be together and one essentially breaks up with her deceased boyfriend to be with the new beau (complicated as you can tell). My belief was that a reader, after having ridden the rollercoaster of a story, deserved seeing the characters go off together in a Disney ending. Sappy? Predictable? Perhaps, but the ending makes me smile and reader feedback supports my decision. The other story is about two people who are from vastly different backgrounds. While my female protagonist dreams of a happily-ever-after she ultimately is wise enough to see that this particular situation/love interest is wrong for her. To stay would be to use the other person, something she has always done and no longer wants to do. The final paragraph is her trying to make the decision to stay or go, and it is a very bittersweet conclusion to the book. It was not how I envisioned the ending when I was writing the story but I wrote it that way because it just felt right. Follow your gut and let the story take it's natural course. The right ending will usually present itself. If you are still unsure, write multiple endings and then let them sit for awhile. Come back in a month and read them again. The right one should be obvious.