I feel so frustrated because I can't change the way I feel. There was this girl whom I really liked. We used to hang out and stuff. Maybe I was obsessed with her. A friend knew I liked her and asked her out, and now they are in a relationship. I am so hurt because of this. Is it a sin for me to want to find a prettier girl or a girl that is just as pretty because I feel humiliated and I feel inferior to them? It will make me feel better or lift up my self-esteem, if I found myself a girl who is just as pretty and godly as her. It won't feel as bad for losing her. I tried changing these feelings, but I just can't.
What you feeling is envy. Envy is wanting what someone else has -- thinking that it should belong to you instead of the other person. Envy destroys a person inside out. "A sound heart is life to the body, but envy is rottenness to the bones" (Proverbs 14:30). It is a sin because it is harmful. "Let us walk properly, as in the day, not in revelry and drunkenness, not in lewdness and lust, not in strife and envy. But put on the Lord Jesus Christ, and make no provision for the flesh, to fulfill its lusts" (Romans 13:13-14).
Your pride was wounded. Not only did the girl you were infatuated with reject you, she picked a friend of yours over you. "For all that is in the world -- the lust of the flesh, the lust of the eyes, and the pride of life -- is not of the Father but is of the world" (I John 2:16). Self-esteem is not necessarily a good thing because it too often means having an inflated opinion of yourself.
Relationships develop when two people mutually decide they want to spend time with other person. When someone picks a person, it doesn't mean the ones not picked are bad. It only means the person decide that another person a better match to them. For example, when you apply for a job and someone else gets hired, it doesn't mean you are a lousy person, it only means the manager thought another person would suit their needs better. In other words, you are seeing this as a rejection of you, but it is not. What it really is an acceptance of your friend.
Getting a prettier girl isn't going to make a bit of difference to your friend. I doubt he is dating this girl because of her looks -- if he is, the relationship won't last anyway. The problem is that you are looking for revenge when no harm was done to you -- except in your own mind. You should be on the look out for a girl whom you love to be around and whom you see as being your best friend. If she happens to be pretty, well, that is just a bonus. But at the same time the girl should be looking at you as her best friend.