I have done a lot of single travel, including three months in Switzerland long ago, and I have experienced depressions such as you are experiencing. They never lasted for long, at least for me. Hopefully, yours will pass soon as well.
The above suggestion of keeping a journal is an excellent one. Or, as I often do, write long detailed letters to friends or family telling them about what you have been doing. While out sightseeing, I'm thinking about how I will describe my day's experiences in my journal (or letters) that evening when I get back to the hotel. I have discovered that, for me, the long lonely evenings in the hotel room are what tend to trigger my bouts of sadness, and the time spent writing takes my mind off that, helps the time to pass more quickly, and reminds me of the travel experiences I had that day.
Another method that always worked for me, perhaps because I enjoy photography, is to take pictures of your surroundings. When I am doing this, I pay much more attention to where I am and what I'm seeing, and I am busy thinking about which angles, etc., are best for capturing a certain subject on film rather than thinking about being homesick.
As mentioned earlier, it seems to be a mood thing with me as well, and breaking that mood, focusing on other things, is what does the trick.