Telling yourself in particular "I am Beautiful" seems a little oddly specific.
If you want to build a tower, you have to start on the foundation first.
Negativity feeds negativity, positive feeds positive. I know, that's a real "duh" statement, but it's true.
I good way to make yourself feel a bit better is to have someone smile at you.
The best way to get someone to smile at them, is to smile first at them. Give it a try the next time you are at the supermarket.
Train yourself to keep away from negative words. Retrain your vocabulary, and it will assist you in retraining your mind.
Lets say your friend is causing you trouble, saying bad things behind your back.
If you say "I have a problem I have to deal with."
you are saying in emotional words: "I have a negative thing, and nothing is good about it."
instead try "challenge"
If you say "I have a challenge I have to rise to."
You are saying in emotional words "I have a goal to change my situation with my friend, and it's something I can win, there is a prize at the end."
If you say "I have an opportunity to change this with my friend"
You are saying in emotional words "I get a chance to do something I would not have been able to do when it comes to dealing with my friend, and I should be glad of it!".
It takes a while, but makes a difference.