Certified GKH Gardening Expert
Many apples and cherries (which many are barely hardy in your zone) are not suited for zone 8. This may be why you are having a hard time. It will be best to go by the USDA hardiness zone listed for the specific cultivar that you want, as it will tell you if it is able to survive in your zone.
Even then, cherries will have a hard time if not planted where they receive some shade in the hot afternoon. It will be necessary to protect plants like this from the heat with a shade cloth.
In short, it will be best to match your USDA zone with the plants' USDA zone ranges. It can help to make sure that the plants' zones extend past your own in both directions, when possible. This will ensure better survival rate.
Here are some articles that will help you to find trees for your area: