This could be because she's not the right one, or it could be because (regardless of the partner) you just aren't ready.
It might be helpful to take some time to think about if either of these things ring true for you. If they do, then you can decide what you want to do about it.
If you genuinely feel the time and person are right, then it might be helpful to take some action. If things are working well until she touches you, why not take control? You touch HER, tell her what you like, etc. Also try to really focus on what is happening rather than worry about losing your erection (easier said than done, I know).
But I will say that worry or anxiety can cause you to lose an erection, just as they can make it difficult for a woman to reach orgasm.
Take things at your own pace, try to enjoy what's happening, and see what happens. If you only have problems in this area with other people, then chances are something emotional is the cause. (Unless you're drinking a lot - heavy alcohol consumption makes it tricky to stay hard.)
If you regularly have trouble achieving or maintaining an erection, there might be a physical problem and I'd suggest you see your doctor.
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