Where you are right now is a result of what you have been asking for with your thoughts and actions. If you're not pleased with your present situation, and you feel stuck there, then you need to adjust your patterns of thought and belief.
If a pattern had been with you for a very long time, it can be difficult to break. But if you want your reality to change, you must replace your current patterns with new ones. Sometimes it can be helpful to identify the negatives thoughts, but you have to be careful here. You don't want to fight against negative thoughts - this can often create even more resistance. Did you ever try to make yourself stop thinking about a certain subject? It's pretty difficult. It's a much better idea to try to think of other more positive things.
The best thing is to listen to your feelings - they will always tell you whether what you are thinking about will lead to happiness. When you get negative feelings you should immediately find something positive to focus on. You don't need to worry about the thoughts you were thinking, just start thinking about something else - immediately.
Just as pain in your hand tells you to pull it out of the fire, negative emotions tell you to stop focusing on your current thoughts, because these are causing you harm. Your negative thoughts will end up attracting more negative things into your life. Bad feelings are a warning sign that you're going towards a future you will not enjoy. Heed the warning - stop where you are, and start moving in the other direction.
Suppose you are thinking about work, and you realize you're feeling agitated and annoyed. Listen to those feelings and start focusing your attention on something that you know makes you feel good. You might want to have a mental list of things prepared for these occasions. Think about a loved one, a favorite pet, an upcoming vacation, a funny movie, etc. Just get your mind moving in a more positive direction.
It isn't necessary to spend time trying to analyze your thoughts or push negative thoughts out of your head. All you need to know is that these thoughts are not serving you well. However, sometimes there's so much momentum built up in one direction that it's hard to suddenly stop. In these cases you might try to "soften" the situation first by at least slowing down the forward movement.
If you were imagining worst-case scenarios, start focusing on the best possible outcomes; if you were picturing a co-worker at his worst, think about him at his best. You'll know you are "slowing down" when you feel some relief. Then look for thoughts that bring a little more relief. When you sense that you are coming to a complete stop, start putting your attention on purely positive things, and start moving in a different direction - towards happiness. This is your new course. As long as your feelings remain joyful, you'll know you're heading in the right direction.