It really starts with your awareness. What type of thoughts are going on inside your head? Are they negative or positive? Now ask yourself which one takes up the majority of your thinking time. Know that your thoughts affect your entire body, your mood and your actions.
Now knowing this you can take control and take action. I suggest just like brushing your teeth everyday allocate yourself a little time to having those negative thoughts. Then after your allocated time is up, say to yourself or say it out loud, well that’s enough of my time given to the negatives today. Then move to the positives, maybe you could start with writing a list of things you’re grateful for and add to it daily. Practice this daily and notice how your feeling in a few weeks time