Q.peach tree
I have a 4 year old peach tree that seems to be growing well. It produces a lot of fruit, but they are the size of golf balls and will not get any bigger. What am I doing wrong? I tried pruning it back in the fall and I tried thinning them out when they first started budding, but nothing seems to work. They grow fine but never get bigger than a golf ball. Can you please help me, or is it a lost cause?
Thank you very much in advance.

Certified GKH Gardening Expert
Thinning is the critical step to prevent this problem:
https://www.gardeningknowhow.com/edible/fruits/peach/peach-tree-thinning.htm
http://www.clemson.edu/extension/peach/faq/when_can_peach_thinning_be_done_and_how_when_is_the_best_time_and_why_are_there_any_potential_problems.html
If you are already thinning enough and at the right time, then it could be a problem with watering, fertilizer, or disease. Check carefully for any signs of insect or other infestation. See these articles for advice on peach watering and fertilization:
https://extension.missouri.edu/p/G6030
http://homeorchard.ucanr.edu/Fruits_&_Nuts/Peach/