Hello Dear and Welcome to the Urology Forum.
Oh my!! Don't even think about injecting anything into your penis. It's dangerous kiddo! You are most likely through puberty at 19, unless you are very, very latent.
Most guys start puberty between ages 10 and 14. But, puberty happens at different times for everyone. You may have a penis that's grown to its full size as early as age 13 or as late as age 18. Everything I read say the final stage (not the onset) of male puberty occurs anywhere from age 14 to age 18.
Remember there is no such thing as a "normal" penis size. The size of your penis depends on your genes, just like the size of your hands, feet, and eye color. Keep in mind, penis size has nothing to do with manliness or sexual ability. I know this to be a fact. Some of the best partners I've had were men with smaller penises.
What you need to know is the brain is the largest sex organ we have - it's not our genitalia. I don't mean to be crass but it is true - it is not what you have but how you use it. If you don't learn that - you will never know how to truly please your sexual partner. Again I know that from experience - and all my friends will say the same.
My friend you simply cannot change the size of your penis. Despite what you may hear, no exercises, vitamins, quick fix, injections or diets can change the size of your penis - or make it grow faster. What you read on the internet is basically not true regarding increasing your size. However companies make a lot of money by claiming otherwise. They prey on young men such as yourself who unjustly believe they must have a larger penis - sometimes even those that measure 7.5inches - everyone wants to be larger. Don't be foolish my young friend - there is so much more to sex (and life) than the size of one's penis.
You say you don't want to lose hope - but don't place you hope in the wrong place. There is no reason to have low self esteem. Learn to be a masterful love maker and you'll never have a complaint regarding your size. Sounds like a fairy tale? It's not, it's true.
Sorry dear. I know I didn't give you the answer you wanted. I did give you some magic - read what I've written - that's the magic.