What is wrong with my poor little lemon tree?

Citrus fruit likes to be soaked. You should only need to water it every 10 days or so, soaking the soil each time. With the tree in a pot, let the water run out of the bottom. If you weather in exceptionally warm, you will probably need to water it a little more. You might also check the stems to the leaves, there is a "critter' called scale that likes to invade citrus and other trees. They look like little green bumps, and can be controlled by spraying. If you have had winds lately, they fruit may have dropped off, as well.