When my wife and son go to school and work, and I go to work, I feel lonely and depressed the whole week. Why?

My weekends are better. I love spending time with them, but when Sunday rolls around, I'm depressed and sad. Monday morning is horrible. I cry a lot, even though I pray. I'm not wild about my job, but it's a good one, and I am busy with three other professional projects that get a lot of attention and compliments from the public. You would think I have a lot to be happy about, by all I keep thinking about is falling off a cliff. I just don't feel accomplished or successful, and I'm VERY sad when my wife and son leave the house in the morning, even though I'm a few minutes behind them. If I get home a few minutes before them in the afternoon, I am very eager and happy as I wait for them, but, in the morning, as they leave, the idea that it'll be 8 or more hours before I see them really depresses me. Even when I get home, I'm not in the best of moods, although I'm happy to see them. It's just the day, and the mundane nature of work. Again, I wish I could work from home, making a really good living. I really believe that kind of day would bring me more joy and happiness because the idea of it lifts my spirits a bit. As it stands, the sadness and lonliness throughout the day gets so bad that it actually hurts, physically.