Men tend to misjudge the size of their penis, due to viewing it from above. Many men believe that their penis is smaller than normal. You could also measure the size wrongly.
It is shameful of your girlfriend to make fun of your penis size. It means she is not serious about you and you should reconsider your relationship. A penis is the most precious thing a man has and he will see and touch it many times a day!
Since you are now 19, it is possible that your penis may still grow an inch or two. Penis growth usually starts between the ages of 10 to 14 years of age. If you are still gaining height then the penis size may still increase though.
The penis is capable of lengthening and increasing in overall size during sexual arousal. The longest fully erect penis (scientifically validated) is 12 inches (30cms). The smallest normally functioning penis ½ inch (1.3cms). The average erect penis size is around 6 inches.
This means you have an adequate size, but you need to learn the art of making love with it. There is no reliable scientific or credible evidence that pills, potions, or lotions work to enlarge the penis permanently. So please do not fall into the trap in the media or the net that promises to increase your penis size.
If you are wondering whether there are, any surgical procedures to enlarge the penis, yes there are surgical techniques that can increase the girth and length of the penis. A ligament that holds the penis in place can be released extending the penis by about an inch.
Dermal implanting can add length and girth. As with any surgical intervention you must consider whether the perceived gains are worth the risk. There are a number of side effects and the cosmetic result can look odd. Many urologists will not perform this type of operation unless there is a deformity or some other mal-function.
Fortunately, penis size is not related to sexual satisfaction. And this I presume is the crux of your question. Spend considerable time in foreplay. Make your partner's orgasm your focus. Penis is not the only thing you have, you possess beautiful long fingers, a flexible tongue. Use these to bring your partner to orgasm first, then penetrate and she won’t even complain about your small penis. Learn the start - stop method to control your urge to ejaculate.
Love making is an art. The penis is only an accessory. Learn the art not from books or this answer, but by practice, experimenting and imagination. All the Best. Smile.