Just like there are steps for MPUA, there are steps to mastering great sex and staying hard.
Do you last that long because you can't stay hard or is this is only when you try and have sex or maybe because you have a small penis and your embarrassed about it? Whenever we have problem we have to be HONEST with ourselves in order to fix the problem. I know how it is I've had problems staying hard with many prior girls, so I evaluated my problems to find out that I just wasn't doing the right techniques to relax. I was over thinking the situation rather than relaxing and enjoying the moment, I wasn't following the steps to successfully stay hard AND give the girls maximum pleasure that leads them to many multiple orgasms. Once I learned these steps and techniques though, my sex life changed dramatically all of a sudden girls needed me for sex. Hope I'm a bit a of help brother

If she has the impression that you have had sex with other girls then A) You be honest with her and tell her that your a virgin (truth hurts sometimes) or B) Study your ass off on pleasuring women, foreplay, how to eat her out (oral sex), the vagina, giving her multiple orgasms, g-spot, breathing techniques, meditation(I seriously thank meditation for my success with women), the sexual psychology of women and stimulation spots in the women's body to turn them on. If you get soft your probably over thinking to much and not relaxing enough. Just try and relax before you know your going to have sex tell yourself that you will dominate her and she will be addicted to you for sex. BREATHING IS KEY!! BREATH THROUGH YOUR NOSE AND OUT YOUR MOUTH DEEP DEEP DEEP BREATHES. You must must must learn breathing techniques that help you perform better and exercise daily for at least 15 min to improve your stamina. Be confident of yourself and you need to make sure that you don't "climax" during your masturbation time for a good 4-5 days I would say. Play with yourself during those 4-5 days but DON'T ejaculate just before your about to "climax" let go of your penis for 10 seconds (BREATH!) and repeat couple more times. This will help you out to bust one faster plus you will also feel confidence to know that your penis will be ready for action because of the sensitivity it has.
Before you have actual sex do some foreplay for a good 25 minutes and have fun with it. Learn were the women's stimulation spots in the body are to turn them on faster are and at the same time the vagina, explore it. Google is the best tool in the world use it brother trust me you can literally find anything online. I would recommend you LEARN and MASTER the basic techniques a step at a time don't overwhelm yourself with everything, but be consistent on expanding your knowledge because after all that is how we LEARN to become MASTERS.
Study study study.... practice practice practice. Remember your results will be on how determined you are to find out your problem and what type of action and time your willing to put in it. Very simple

ohh ya no more sex in shower just stick with the bed or something comfortable remember you want to set the feeling so maybe be romantic buy candles along with some tasty sour watermelon lube, girls are suckers for shit like that so it helps a lot of times to be creative.