First, you need to set your mind to feeling better. If a person is always saying how sad they feel, it's really hard for them to get better. I'm not saying they won't, but it is much harder, so set yourself in the right frame of mind, tell yourself you feel great, even if you don't yet. There is an old saying, "What you say across your soul, will come true", this was said about people who made comments like, "I think I'm coming down with a cold, then they do". You must tell yourself everyday as often as possible, that you feel great.
Put pictures up in the bathroom, in the car, on the refrigerator, anywhere you spend a lot of time, even at work if that can be done, of people smiling, or things that make you smile, or feel happy.
Things that make you feel bad, keep out of your mind. This sometimes is easier said then done, but can be done. It will get easier as time goes by. When you get thoughts about bills, bad times, or anything that gets you sad, put it out of your mind right away, don't give it any thought at all. Find something to do, or pray, sing, recite funny sayings, anything to get your mind in a happier place and off of your troubles.
When you only think about the bad things, or what may happen, what you think you should have done, about bills, etc., you will set yourself up to be depressed, but if you make an effort to not think about those things you can change how you feel. Remember you can't change the past, it's history, you can't worry about the future, it's not here yet, so you can't change things there yet, you have only today which is called the present for a reason, it's a gift to you, ie. the present, so enjoy it to the fullest. You can only do that if you are happy. The happier you are the better you can deal with things as they come.