I have a crack in my drywall leading from the structural ceiling beam to the corner of the fireplace mantel?

It is approximately four feet in length and is sort of a gradual slope from one to the other. I have tried, unsuccessfully to patch it with the mesh plastic tape. I did make a groove in the wall in the crack like a ditch, my dads advice. But it still re-cracks over time. What would be the RIGHT WAY to fix this issue? Im tired of looking at it and im sure when i sell the house people will say "look at that crack, yikes". Thanks for your advice...