How can I last more?

Ive been in a relationship for a year now where we were both virgins, we are both 18. We had sex for the first time a few months ago after months of just blowjobs and handjobs. I felt very confident about my “endurance” because the first times we had sex she came very fast and I didn’t. It even became a problem for both of us because I wasnt able to cum, she would be for 30-40 minutes doing a hj or bj after she came just to make me finish but she couldn’t. This made me feel bad because she thought she wasnt good enough, but also good because I was able to last that much time and made her cum so fast with oral sex or penetration. But the last time we had sex she was able to last more time (my theory is that maybe she got used to it) and when we were at doggy she kept asking for more and I wasnt able to control myself and ended up cuming. She was so polite with me and told me to not feel bad, I made her cum with oral sex after that but this situation has been on my mind ever since. She likes me to fuck her hard and fast so this makes it harder for me, I try to focus on other things, making variations on the fucking speed haha, stopping to kiss or doing other things and then continue but none of these seem to work. I would like to know if you guys can give me some advice on what to do because I dont want to feel that I am nit good enough just because of this.