Give yourself a goal, a task, or a project to complete and whenever things get tough, condition your mind to focus on obtaining those accomplishments. If school becomes your task, then post pictures of your ultimate career from magazines and pictures from the net on your wall or fridge. Now each time your lows seem to consume you, it'll make you work that much harder to make your dreams come alive. This does take time and some disciplining but well worth it.
For every negative emotion you feel, put it inside of a mental box, label the box and figure out through time and effort what could be done to begin the healing process of getting rid of the box permanently.
For example, if you've just found out that your significant other has cheated on you, of course you feel hurt, sabotaged and betrayed among other emotions. Identify your specific emotions and evaluate a path to recovery whether that involves communication with your mate or with a professional. It may involve spending time a part or even leaving the relationship altogether. Just remember that a step in a better direction is never a wrong one.
Gain control of your mind when negative thoughts begin to surface, quickly replace them with things that are enjoyable and pleasant. This can be accomplished by listening to your favorite recording artist, watching a movie that brings inspiration or talking to someone that keeps you believing you can reach any mountain you stretch your arm toward no matter how high.
Throw yourself in someone else's life who is less fortunate than you. Do something good for a stranger. Volunteer your time to a cause dear to your heart and put all your effort in making a difference.
Change your view point of the world, life, and how you view yourself. Hating these things will only compound the negativity inside. Analyse the place negativity is coming from and decide to control your future by changing your attitude and focusing on the things you love.