I think Lana relates this song to a relationship with a girl. It`s quite simple.

For the ones who don´t know the brand Mountain Dew: It´s a lemonade from Pepsi which is also available without sugar. The fact that this drink is not always avalaible, i.e. only in certain seasons, makes it something special. I think this is the truly message behind this song: not a normal relationship(hetero)but a different relationship with a girl instead with a boy.

"You´re not good for me; baby, you´re not good for me; you´re not good for me; but baby, I want you, I want"

In this part she is offending that their relationship isn´t good (in opinion of most people), what means that still not everyone is supporting LGBT people but she want to be together with her girl.

"Diet mountain dew, baby, New York City; never was there ever a girl so pretty; do you think we´ll be in love forever?; do you think we´ll be in love?"

Lana said that she wrote this song because she was in New York. Maybe she saw there a pretty girl and wished they had a relationship or she already had, who knows. But I think New York is a special city for her.
She is talking about the beauty of her girl here (or a girl she fell in love with). She seems to hope that they are forever in love respectively that she will never break up with her.

"Baby put on heart shaped sunglasses; `cause we gonna take a ride; I´m not gonna listen what the past says; I´ve been waiting up all night"

No boy would wear something like that, so it´s quite clear that she is speaking to a girl. I also think that she had problems to accept that she isn´t hetero and that she is trying to avoid these thoughts because she is happy and just wants to be happy with her girl. And she mentioned that she waited a long time.

"Take another drag turn me to ashes; ready for another lie?; says he´s gonna teach me just what fast is; say it´s gonna be alright"

I think that "turn me to ashes" means something similar to "break me into pieces". Lana thinks she could be hurt or will be hurt. It is also mentioned that they have to lie, maybe because of their relationship. I could imagine that they haven´t told it even friends or family (and of course not the publicity). With "he" she could mean a male friend (or boyfriend?) because I think they don`t got much time to be together and Lana wants her so bad that she know because of him what fast is (to be fast in a new relaionship).

"... can we hit it now low down and gritty?"

I think this is related to sex. She want hard and deep sex (literally: to penetrate deep in her moist middle).

"Let´s take Jesus off the dashboard; got enough on his mind; we both know just what we´re here for; saved too many times; maybe I like this roller coaster; maybe it keeps me high; maybe the speed it brings me closer; I could sparkle up your eye"

In this part she is only referending to their happiness and that she don`t care what the bible says. God accepts everyone just who they are so you don´t have to worry either. With "roller coaster" she means her feelings- she don´t know how to react to this chaos but she seems to feel good with it. With "speed" she could mean that the pace is quite fast (she just had boys and suddenly she fell in love with a girl, she isn´t used to it) so it´s a new experience to her. And she wants the please her, look good for her, etc.

"Baby, stoppin´ at Seven Eleven; there in his white Pontiac heaven"

She means that they go out to Seven Eleven and Pontiac is a city in Michigan.

I hope you like my interpretation of this beautiful song.