Focusing on negative thoughts is unfortunately a common condition. I recommend that you discuss this with your primary care physician
who may be able to help you with this.
In general, the focus on negative thoughts falls into the spectrum that we call "mood disorders." As you mention, depression
(or major depressive disorder as it is known) is the most known type of mood disorder. This involves a constellation, including changes in appetite, sleep patterns, ability to enjoy life and ability to concentrate among other changes. If you do not feel depressed, this is not likely the diagnosis. That being said, a minor depressive disorder could be your diagnosis. This is essentially when people do not meet all the criteria for major depression. This can also be treated. Another diagnosis to consider is dysthymia--this is essentially a mild form of these negative symptoms that people have for greater than two years.
It is important to diagnose these conditions. Addressing the condition is the first way to treat them. In addition, these can be treated with both psychotherapy as well as medications.
There are also organic causes of these feelings. For example. low thyroid levels or high calcium levels can result in these symptoms. These should be ruled out.
Talk to you doctor