Apple (Tomato) Of My Eye

My container garden isn't going like I planned. What once was a site for so much promise is now a reminder of my failure and disappointment. And an eyesore.

I'm not sure where I went wrong. We water. The plants get plenty of sun. We have mild temperatures. Despite all of this, the branches and leaves are turning yellow and breaking off one by one. Thankfully it is bearing fruit. One lone tomato is growing on my pitiful plant.

She's a beaut, ain't she?! I'm awfully proud of that single tomato. Since there is no sign of this plant yielding any other fruit, I gingerly tend to my one precious tomato, watering her ever so gently and praying that the branch holding her up doesn't give way. She's going to be delicious, I can tell!

Any idea why my plant looks like this?