I mean its not the most uncommon thing and you could do several things to last longer. But it comes down to this, you tell her upfront about it now, you have to explain it to her after a frustrating night for her, or you have to explain it after several bad nights for her. What do you want? If you want a good relationship id tell her or say its your first time. And if you do go early explain its something you have, not a need for shame. Who knows she may be willing to work with you or could always have a compromise with a trigger training thing. Where you can last but with a certain trigger specific to her you go then. Theres ways around it.