I am a bit confused as to why you say that you don't think you will ever go to therapy? It is not a sign of weakness to seek help or a diagnosis. Many people live with depression for years, and gradually get worse, because they feel they can "tough it out" and not get the help they need. I feel it is a sign of strength to admit that you are not feeling well, whether it is depression or pessisim as you call it. It is not fun admiting we need help, but depression and other mental illness does not have the stigma it used to - it certainly wouldn't hurt just to go in and get screened or talk to someone. Even if it is "pessimism" you can still get the cognitive therapy to change the way you think.
If you decide for some reason not to seek help (and I hope you change your mind someday) there is an excellent self help workbook (best seller) on changing your thoughts by cognitive therapy, It is called "Ten Days to Self-Esteem - defeat depression and discover the joys in daily living" by David Burns MD. It is a great workbook on pessimism, depression and negative thinking. Good luck!