there valencia oranges

Not ripe yet. Your pushing it. The longer they sit on the branch the more sweeter they get.

Most orange trees use a "sour orange" plant as the main plant, and then they graft on the branches of plants that produce better-tasting oranges. My guess would be that you need to do that.

Maybe they are just not ripe enough? Let them stay on the tree for a few more weeks.

! could be a mistaken (wrong) variety! Peel the oranges cut them into quarters sprinkle salt over them, that way it cuts off the sourness! or you can make a sugar solution to your desired taste and pour over the cut pieces of orange enough to cover them and leave in fridge for an hour or more! You dont need to buy another tree!!!

Valencias are late season oranges. They will be ripe in March or April. Just be patient.
You might consider planting an early variety ( ex. Hamlin or Navel) and a mid-season variety (ex. Murcott or Temple) so that you will have ripe fruit throughout the season.

http://flcitrus.ifas.ufl.edu/UF%20IFAS%20Short%20Course%20Proceedings/Citrus%20Maturity%20and%20Packinghouse%20procedure s/Commercial%20Varieties.pdf