You’ve gotten some good advice here. The exercising suggestion is a great one, as well as developing a hobby/interest/skill that you can feel positive and good about. Along the same lines, have a few goals in life that you are working towards. If you’re not living a life that makes you feel fulfilled then you will naturally tend towards negative thinking.
Another piece of advice would be to deal with any unresolved emotional issues that may be at the core of the negativity. And begin a meditation practice which could be very helpful in a number of ways. Finally, if you believe in God, pray regularly. Ask for help with your struggles, but also count your blessings and get in touch with feelings of gratitude.
It may benefit you to work with a therapist who is more of a life coach to help you set goals and work toward them. You can do this also by making and keeping a written record of what you are working toward, the steps you are taking to get there, and the progress you are making along the way.
Lastly, get out in nature as much as possible! It’s hard to feel morose when in the presence of fresh air and natural beauty.