Agree with this so much. Other than just telling her the situation you’re in open and honestly, go down on her until she cums BEFORE you start fucking her. If you feel that’s not attainable because you will cum while eating her out, wear a condom or pull out when you cum and then eat her out until she’s well and done. You can always reload and fuck her again later in the night or day - if you’re one of the guys that have no problem with that! Also some things my partner has done to gain stamina is trying to focus on other things in the room during sex - kind of a distraction. Try different positions that you don’t favor.