Most people (even those who live on the equator) actually have vit d deficiencies. @_@!!

Another thing you may try doing is writing something positive on a piece of paper. (3 positive thoughts? 3 motivators?) Put them in your pocket and when you get the negative thought let it come in and out of you while touching the paper. The paper represents the positive energy as you experience the thought. (There are a lot of variations to this.) It takes a lot of practice because its hard not to pay attention to those thoughts. Failing is a part of getting better though.