I don't know why you would be inclined to write an ending you don't agree with in the first place. Have you considered not giving them a happy ending? Personally I think that would be much more interesting.
Well I really suck at finishing stories to the end. I did have a story I wrote with a happy ending but it's basically because I planned it that way from the beginning. I guess when you're thinking about the problems that the couple will face, don't create some kind of problem that would be impossible to solve. Try to think of the problem and the solution at the same time but the trick is to not let the characters realize there IS a solution to their problems until they've struggled a bit.
Life (and relationships) is full of ups and downs. End the story on an up. That's really all you need to do.
If having all the issues solved by the end of the book and the characters being happy as they can be isn't 'realistic' enough for you, try writing an ending that implies there are still more problems to come but the couple have gotten over the biggest issues and are at a happy place for the moment. Leaving the reader with a sense of hope that things will work out for the characters from that point on can count as a happy ending, even if there's still a chance for things to take the turn for the worse.
Never conform yourself to an ending, happy or sad.
I like to think of my characters like lab rats... In my mind they are there but then a toss a bunch of bad stuff at them, make them walk on fire, go through a hurricane, and deal with societal issue from hell. I watch and observe them then write down the results.. when I start writing a story I don't always know how it ends or if it ends... Yes, I'm sucker for creating cliffhangers.
when writing a romance story how can make the two chacters in my story have there happly ever after when i'ma realist? any tips
Happy endings don't have to be saccharine, nothing-bad-will-ever-happen-again parades into the sunset. Maybe not everything is perfect at the end, but they'll try and figure it out.
People being together long-term isn't unrealistic.