For as he thinks in his heart, so is he. —Proverbs 23:7
I encourage you to practice being a positive person. It's just a matter of breaking one bad habit and forming a new one. I was so negative at one time in my life that if I even tried to think two positive thoughts in a row my brain got into a cramp. But now I am very positive and actually don't enjoy being with people who are negative.
Discipline is required any time you are forming a new habit. You might consider putting some reminders around your house or in your car, like little signs that say "Be positive." Ask a good friend or spouse to remind you if they hear you slipping into negativism.
Practice trusting yourself rather than doubting yourself. If you are applying for a promotion at work, don't think to yourself or say, "I probably won't get it." Pray and ask God to give you favor with your employer and then say, "I believe I will get the job!" And if you try and the outcome isn't what you were hoping for, then tell yourself, "If the job was right for me, God would give it to me, and since He didn't, He must have something even better in mind for me." You can train yourself to be positive in what appears to be a negative situation.
Lord, show me where I am stuck in negativity and need to break through to trusting You. Help me to have the right thoughts and attitudes that move me forward. Amen.