Setting a positive intention can help, but if you continue to think about these negative things, you send confusing signals to the universe about that which you desire. So, if you are setting the intention, "I choose to be vibrantly healthy," but you continuously think about how sick you are, or if you set the intention, "I am financially prosperous," but continue to worry about how your bills will be paid, then your negative thoughts and focus are overriding (or at the very least cancelling) your positive intention.
When everything is going great and you're feeling positive, it seems pretty easy to set intention and focus positive thought and emotion to create, but what do you do when something negative happens or you are in a negative place? How do you direct positive emotion and thoughts when you're not feeling positive? How do you turn a situation around when what you perceive as your reality (being sick, being unable to pay your bills, etc.) is in direct conflict to your intention?
It's all about how you focus. If you continue to focus on the problem, then you are creating the energetic conditions to perpetuate the problem; you are calling it to you through your emotion and attention. To reverse this energy, focus instead on the solution. Visualize yourself with the problem solved. So instead of focusing on the belief or thought, "I am so sick - look at all the evidence pointing to how sick I am," visualize yourself as being healthy and healed. Visualize yourself as being vibrant and whole. And when the inevitable thoughts of the problem arise again, take a moment, close your eyes, cancel that thought, and visualize the solution. Do this for just a moment every time the negative thought arises.
So when the thought arises, "I can't pay my bills," for instance, every time you catch yourself worrying about how you are going to pay your bills or thinking something along the lines of, "I don't have the money for this," stop and briefly visualize yourself easily paying your bills or with a bank account full of cash (or both). Shift your focus from problem to solution, and do it consistently. When you combine this with your intention, you can begin to create that which you want instead of that which you don't.