Give it as much sunshine as you can, and try not to over water, but don't let it dry out. There are specialist feeds, but not worth the money for just one tree, try some slow release pellets they should see it fertilised until August and then if you can find some 0-10-10 fertiliser that will help it through the winter if you feed fortnightly through August and September. You may have to search for it, but it does help.
The old leaves only last a year, so they do fall off, only fake plants are capable of keeping all their leaves, and only when not exposed to sunlight or bad handling.