That means talking. Lots of it. You may feel you've already done this, but it may be you're avoiding the hard bits because it seems to be true that you're saying it isn't possible for you to also choose to not be sexual, and fair enough. This creates a stand-off - and I get you can feel stuck there.
The only way your partner is going to know how important this is to you is to be clear with her, and that you both commit to exploring all options to solving the problem, and staying together. There may, for instance, be options she has not considered for herself that may help her with whatever physical or emotional blocks she's experiencing to sexual intimacy. I think these need to be exhausted first - if she's willing.
If you do decide to go ahead - with consent - with having sex with other people, then just like any non-monogamous relationship the "contract" is vital. You need to both be clear about what the ground rules are, how it will work, what is okay and what isn't. You also need to be committed to keeping talking about the agreement, and that either of you can veto it at any time.
And be aware that having sex with other people might not be the clear solution you think. It's natural, especially when you've been in a long-term relationship, to strongly associate sexual intimacy with emotional intimacy - the last thing you want to do is fall in love with someone else.
Be prepared for the reality to be much more complicated than the fantasy. And be prepared to talk about that too.
Q: One of my best friends, who I have known for a long time, always makes everything about her. When I try and talk about my life, somehow we always end up talking about her life and her problems. Is it wrong to raise this with her?
A: Such an annoying habit isn't it. I'm not surprised you want to raise it with her, however, some things to think about first.
You've known her for a while, so you'll likely be able to tell how much insight she has about this. It's actually not that uncommon, as we try to relate to others, to try and locate situations or experiences that enable us to understand what people are telling us.
But it ends up looking like just being a bad listener.
If we assume the best, it's likely she's trying to connect with you and even help and I don't think there is anything wrong with giving her some clear and kind feedback.
Generally, if it's not too heated then it's best to raise it when it's happening and to start by validating her, and then ask for what you want instead - "I really appreciate you're connecting with what I'm talking about, but I find it hard to finish my train of thought when you turn the conversation around to you. Can I just finish what I was saying?"
Above all, with feedback, be kind and stick to the facts. It can be hard to hear what other people think of us, and the chance she will hear it is higher if she's not on the defensive.