I try to stay in the moment. I try not to dwell on the past or the future. If the past and present come to my mind with negativity; being in the present moment helps me stay clear headed and I am more aware of any negative self talk. If my mind wanders to things I wish I did different, I stop myself and say, "That moment is over, what can I do now?"
Sometimes when the negative thoughts come into my mind, making me less positive, I also accept the thoughts, let them pass but I don't let them define me. I tell myself, nothing is a mistake it's a lesson. I even tell myself that I love me.
One thing that seemed to always help was, while going to sleep and while getting ready for work the next morning, I would say to myself, "I am going to have a good day." And I would.
It doesn't mean to deny any negative thoughts in your mind, it's about acknowledging them, letting them pass and moving forward. The biggest key to staying positive is to pay attention to all your thoughts and stopping yourself from beating yourself up.
There will be plenty of times, when you wish to hear things positive from someone else, in an attempt to feel self worth and positivity. For example, I love you, you're special, I care about you. But when they don't come from anyone, when you feel you might need that, you can tell yourself those things all the time. You don't need to wait to hear it from someone else.