AmberSpyglass
If it’s not something you want to discuss, just politely say that. There’s no need to be rude - there’s nothing immoral or shameful about our sex lives, but it’s fine if you don’t want to discuss it. Try and do it without embarrassing her or seeming judgemental though.
A fair response.
Depending on how close the friendship is and what two friends have mutually agreed to discuss either tacitly or by design, the rule of 'consent' also applies here.
We all know that sex is 'natural' and normal. But to a large extent it is still very personal... your tastes, likes and dislikes. Everyone is different.
So to some extent, talking about sex is like the physical act itself and if you are not close enough to the person who is sharing their experiences - and by that I mean close enough to know that the other person won't be offended or be made to feel uncomfortable with what you tell them, you really need to get their permission first. And it can be done quite tactfully, ie, 'do you mind if I share something quite personal with you about my sex life that's bothering me?'
Obviously, if you are really close friends and know each other extremely well, that might not even be necessary, but I get the impression that although the OP has known this person for 10 years and they are "good friends" they are not really 'close' friends in that sense - otherwise the other person would know that what she was saying was making the OP uncomfortable.
There are people who are paid - or who volunteer - to counsel on sexual problems. I doubt anything shocks them or makes them uncomfortable, and they are the best people to talk to - not least because they can offer friendly and practical advice in a non-judgmental environment - and make you realise that your problem, whatever it is, is not unusual nor strange. But we are not all made of such stuff, and a cringing friend is going to be of no use whatsoever.