Usually cumming early is a sign of nerves or overexcitement.

Like you're in awe by the fact that you're even having sex.

So chill out, deep breaths, and don't think of the girl in bed as anything special... just another conquest, no big deal.

Mentally you must have the right mindset.

Which is that she's nothing, you're doing her a favor by fucking her, not the other way round.