"Also with an attitude like "I know I cant please her" you definitely won't. Have some confidence in yourself. There's more to sex than cock in cunt." I cannot stress the importance of self-confidence (in any given area of life) enough.
This mindset (and all insecurities you may possess) is (are) incredibly transparent to anyone exposed to it.
In fairness, I understand that, considering your physique in the past, you may have difficulties projecting assertiveness- which is, of course, completely normal. After all, how one carries themselves isn't solely based on what they carry upstairs, but the entire package (body, and any parts of it), so to speak.
If you are making dramatic adjustments to your lifestyle, it is essential to keep your mind and body on the same plane. As you grow older you must remember, the narrative is constantly changing. How would you introduce yourself? How have you introduced yourself in this thread? "I'm _______, a nineteen year old virgin with insecurities." First of all, you're not the first, nor will you be the last (and you're far from alone). You're nineteen, this is a transition period in your life, relax and enjoy yourself (simple advice, yes, but apparently very difficult to apply for some). "I'm _________, a nineteen year old virgin working on improving myself and my life (and girls are 'checking' me out!)" will be a far more beneficial mindset for you, and most importantly, it's the truth. I should mention, the above isn't in reference to this particular thread- I am aware that the manner in which you introduced yourself here was to provide us with an understanding of your situation.
If any woman demonstrates any form of interest in you (and vice versa), share more of yourself, if you still have her attention, and her, yours- SEAL THE DEAL. That's not to say, try to "fuck" her, but give yourselves the opportunity to see where this mutual attraction can lead to. She's expressed she wants a sexual relationship of some sort, she clearly finds you desirable.
However, when you (let's be trite once again) do the deed, don't think to yourself "I can't please her", because if you truly believe that, isn't it a tad bit selfish? (I know that isn't your intention, of course.) Having said that, just because you don't necessarily know how to do something, doesn't always guarantee you're incapable of doing so, and, again, vice versa. It's a two way street, amigo. (Incoming cliche!) Lose yourself in the moment, feel her kiss, gently and lightly grab the side of her face, whilst caressing the back of the ear(s), her hair, and small little pecks from the neck to the shoulder(s). Do this compassionately and with sincerity. Everything will fall into place. Be generous. Hell, just be grateful and appreciative- it will show. Smile.
The size of a man's penis is often his "grain of sand", that is to say, the deciding variable which provides the mold for how one perceives themselves (sexually). Don't let that be determining factor of any sexual encounter, especially if you're sporting 6.5x 5.3/6.8x 5.7! You're above average, certainly you must know this already.
If it's any consolation when I lost my virginity I was nervous beyond belief. I just cared about the person well enough to just let myself go entirely and the rest followed. We had sex 13 times (perhaps more) over the course of one night and a day- I assume you're capable of the same your first time around (that's just how excited you shall be after a successful "first go". Trust me, you'll be fine.
I wish you the best of luck, mate, although I doubt you'll actually need it. Looking forward to 'hearing' how this all unravels.