Every thought you have carries behind it energy that you put out into the world.
You literally manifest your reality by the thoughts that you put out into the world and the energy that surrounds those thoughts.
For example, if you are thinking 'people don't like me for some reason' then people aren't going to like you. First, there's the belief, and then there's the low vibing energy behind it. People sense it. The universe knows it. Ask and you shall receive. The universe is always listening so be careful what you're putting out.
If you walked up to me with the thought she's not going to like me', your energy is going to be different and I'll sense it. You'll probably protect yourself either by your resting bitch face to prove that you are unaffected by my 'dislike' towards you, even though deep down you seek my approval. OR, you will be closed off, hunched over, not talkative, and I'll think you dislike me therefore I wont put any effort into getting to know you. Then you'll say to yourself 'see, I was right, people don't like me' thus continuing your isolation from the world and misery inside your limiting belief jail.
ALL BECAUSE OF A THOUGHT YOU HAD THAT WAS COMPLETELY MADE UP IN YOUR HEAD BEFORE THE EVENT EVEN HAPPENED.
You created your reality.
Watch your thoughts. Bring awareness to the negative ones that come up and reword them into positive beliefs. "I am loved" "other people think I'm amazing" "I am worth this friendship" "I am worthy".
Positive reinforcing affirmations and mantras are just the beginning of the practice involved with rewriting your limiting beliefs and thought patterns. But it's in the practice where the change happens. If you don't start, you won't change.