Wear a condom. As others have pointed out, sex without protection feel SO much better, it's a world of difference and if you haven't had this problem before that's probably why you had it this last time. Put the condom on, and you probably will be able to last as long as you did prior to this. You should do it just to be safe anyway.
Also, try focusing on giving your partner pleasure and shifting your focus away from what you are feeling. If you're only concentrating on how you feel you are going to climax quickly unless you're intentionally trying to hold off, and unless you have practice with that you may not be able to.
Speaking of, masturbate. It's a good way to get to know yourself sexually. Try to hold off for as long as you possibly can. Practice bringing yourself to your threshold, so you know where it is, and then backing down a bit so that you don't actually climax. See how long you can go.
