I am trying to have sex for the first time but I am getting premature ejaculatio!

I am trying to have sex for the first time but I am getting premature ejaculation?

many times but not every time when she tries to give me oral sex
i ejaculate immediately in few seconds or when ever she takes my penis near her vagina and touch it with my sex organ same happens most of the time , only some times this doesn't happens
i have a habit of Masturbating ,is it reason why this is happening
but i masturbate i do not ejaculate quickly.
some time i lose my erection just before sex, i always worry that now i am going to ejaculate and what is going to happen.
help me please .

Sex is a learned art.
Ejaculation is an inherited habit.
You have to try this for many times and then you are sure to succeed.
Do not get confused,disturbed and embarrassed.
Try and stop not till you succeed.