Make a list for yourself. Write down all of the positives that you like about yourself. Make a list of the negatives, too. Then, as you are reading the good and the bad, ask yourself, "Is this truly how I feel, or has someone made me feel this way?"
It wasn't until my 30s that I got serious about my negative thoughts. I decided to tell myself I was worthy enough to have good things happen if I wanted, and that I would have to start telling myself so.