Basically, I have a small (3ft) meyer lemon tree that used to be in a pot and has dropped most of its leaves over the last few weeks, after attempts at rehabilitation. I'm clearly not helping, but don't know what to do. Here is the saga in case detail will help:

My sister had the tree in its pot in an extremely hot/dry outdoor environment where it did just fine for about 2 years. She moved and took it to a place where it was cooler and shadier, and the leaves started to turn yellow (toward the outside, with green veins). She gave it extra food in case it was a nutrient deficiency. She watered it somewhat infrequently, although there was a chance that it was a bit damp. After about 9 months, she gave up and decided that maybe it needed to be out of the pot and in real soil if the pot was the problem, and moved it to my house (in N. Cal) to be transplanted. We planted it about 2 months ago. The leaves nearly all had some yellow, but it flowered nicely. A few weeks in, thinking that the yellow was getting worse, I gave it some more citrus-tree food when I watered it one week (watering = filling a trench-y area around the tree once a week). Soon after, some of the veiny parts of the leaves turned brown and shriveled, while the outside parts stayed yellow. Those leaves subsequently fell off, along with all lemonets. Currently, there are about 20 leaves, about 7 of which are a new-leaf green (not too dark) on the bottom branches, but there are no leaves at all on the top.

I don't want to give up on the tree, but I'm not sure what to do to help. Does anyone have advice?

Thanks in advance!!

thanks!