Hi
To make a conscious effort to feel positive you could attempt the following
First overcome the negative emotion (if the same is the prime reason for your inability to be positive)
Try to take your mind of the stress (the problem would remain till you manage to objectively find a solution once you reduce your stress) by deliberately and consciously doing some activity like walking / clearing the clutter on your desk / reading a humorous article or joke
Share your stress with a close friend or partner or spouse or relative. Opening up and sharing always reduces the stress and detoxifies you from negative emotions.
Identify the source of your negative emotion. Many times we do not interpret the issue on hand correctly, jump to extreme conclusions and misunderstand tone and language to mean trouble on hand.
Look at the problem objectively. You would surprised that far too often the negative emotion is linked with the perceived problems, the possible consequences and fear. In reality the negative emotion is you trying to visualize the worst case scenario so that your mind tries to prepare for the worst case scenario.
Next motivate yourself to seek out positives.
Be result focused not problem driven. Change your attitude to see an issue as a challenge that you can meet, not as a problem that you have to cope with.
Visualize the various positive possibilities that would emerge when you overcome the challenges.
Break down challenges into sub challenges and then it would be less daunting and you will suddenly be motivated because the smaller bits are relatively easier to cope with.
Whenever you feel low and a depressing feeling creeping in, imagine the positive comments, the appreciations you have received and the various high points of your life. These will inspire and motivate you and make you feel stronger.
If you have the time and inclination I would also invite you to visit my blog
https://actspot.wordpress.com/tag/be-positive/ ; perhaps you can get some more relevant inputs from it.
Trust these suggestions would help you realize that you have every reason to remain positive.
Best Wishes