At least you know what you limiting belief is. Meditation and affirmations would help you a lot because affirmations will overwrite your beliefs and meditation will relax you and make you more positive.
Try to do what you enjoy and by that I do not mean something addictive as playing computer games. You should try reading more self improvement books because they will uplift you.
If you feel really down, you should look for what good you have in your life and thank for it. The feeling of gratitude always makes you more positive because it raises your vibration and your point of attraction shifts - after some time - and you start attracting more positive things in your life.