You need to get a hold of yourself (and, no I don't mean your penis) and realize that you are absolutely normal. The average size of an adult male's erect penis is six inches. You are only fifteen, and you will probably continue growing until you are nineteen or so. So, even if you never grew any larger, you would still be fine.
Before you whip yourself into a frenzy, I think need to step back and view your obsession with your 'small'penis as a symptom of a deeper issue: you don't feel good enough about yourself, and so you are ragging on your inadequate penis instead of facing the real issue, that you don't think you are all right.
I know you have convinced yourself that if only you were larger, you'd feel O. K. . Even if you had a gigundo penis, you'd still find something else to rag on yourself about, trust me.
If I were you, I would grab the bull by the horns, if I may say, and face the real issue head on (no pun intended). When you figure out why you don't feel good enough about yourself, and resolve the real issue, you will make peace with your very normal penis.
In order to help you figure out where your low self-esteem stems from, you need to look back to your formative years. There are three likely causes for your low self-esteem: 1) Someone put you down, and their negative comments became a part of your own self-concept; 2) You are very angry and that anger is being turned back on the self in the form of self-attack; and 3) You had a parent who didn't feel good about himself. You identified with that parent and took on your parent's attitudes, making them your own.
Once you figure out where your low self-esteem stems from, your next job is to revamp your view of yourself. Surround yourself with people who tell you good things about yourself, and don't allow yourself to put yourself down. If you begin to rag on yourself, identify the origin (rage turned against the self, copying what your parent did to himself, or simply parroting the bad things that a parent said to you).
When you identify the origin of the put down, then deal with it accordingly. If you are angry, then direct the anger at the appropriate party; if you are mimicking a parent who hated himself, simply becoming aware is the first step to stopping that pattern, and if you are putting yourself down the way a parent put you down way back when, becoming aware and actually facing down the parent in your head by saying, 'You had no right to talk this way to men then and you have no right to do it now'will help.
Little by little, as you become conscious and put your foot down, the pattern of self-attack will stop and your self-esteem will rise. Then, you will stop feeling inadequate and you will also make peace with your penis, which is absolutely fine.