...something with my new girlfriend.
Because of lots of stress during the recent time (and going on) I am in a less potent phase. That means, I can only orgasm while masturbating, not during normal intercourse.
Well, her former boyfriend seems to have been completely impotent (she does not tell that exactly), and so one of her big wishes is that I should cum inside her.
I mean, we are having a lot of fun together, including real sex. My more-or-less-good-hard-on can last for a lot of time, and she orgasms several times, mainly but not only by my well-sophisticated manual and oral stimulation. Just this little detail is missing.
I masturbated in front of her afterwards instead.
Now she is concerned, she was thinking that perhaps she is the reason I cannot cum inside her. She told me that this detail is very important for her. She suggested I´d better not masturbate in front of her because this is giving her the feeling she could be the reason...
I only need stronger stimulation than usual, that normal intercourse cannot give me at the moment.
I have a little fear this all could lead to more, innecessary stress.

I am lucky because we are talking completely open (I always told her she can talk about everything and show her feelings, so do I).
And I do not set myself under pressure with it, because this only makes it worse.
I told her it is a temporary problem, but it probably will go on for a while.

So my question is: What is the probably best way to calm her down? I am not sure if she can believe me that she is not the reason, and that my body is also not the reason.

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If every material thing is gone, my smile and my tears stay left.