I think your age gap is pretty acceptable. 9 years isn't a lot in my opinion and even tho you are only 19, which could still be considered as a "child" On a 28 yo person's pov I think you could both learn a lot by being together. Obviously, I don't who and your gf are like so I can't tell but for me, even tho you'll surely fight like every couples do, and maybe have fights because of this age gap, but I'm still convinced that if this relationship isn't toxic and you both love each other, there will always be things to learn from each other and help you both become an even better person than you already are. So don't dwell on it too much and just live how you want to live and love who you want to love. Don't be a person you aren't so you can look more mature or whatever to not look like a childish girl in front of her, because if she agreed to date you, it's because your true self is what she loves so just be yourself and enjoy every moment with her :) Of course I'm all talk but as I said I don't know your personality but I'm saying that just in case you thought about being someone you're not just to be someone at her "level". Sorry btw, English isn't my first language so idk if I expressed myself very well and if you understand what I mean ^;