I can see this topic has been done over and over again but my case is slightly different than the others. This tree is about 5 years old, my grandmother gave me two seedlings she found inside one grapefruit she had bought. One died in its first year from frost and the one that survived did barely. Unforunately it has been dropping leaves for the last year or so. They turn yellow along the vein and slowly moves out until the leaf drys out and drops. At the end of this last winter it grew a bunch of leaves at the top and a few at the bottom. The main stalk dropped all leaves about 6 or 7 months ago. I haven't been able to identify any pests or disease and attributed it to overwatering, which I used to do once a day until all the soil was visibly moist. It was fine with it until about a year ago. Any help will be greatly appreciated.