Thank you for writing to AlterHéros with your question. You indicate that even though you have been with your partner for almost 10 years, you are no longer attracted to her. Though you say you love her a lot. Does this mean you are emotionally attracted to her, but not sexually attracted to her? Furthermore, are you romantically attracted to her? For many people, it's important to have all of these components in a relationship, though the importance of sexual attraction may diminish as we age or as the relationship ages.
You mention you've had a discussion with her concerning an open relationship, for which she expressed no interest. Before arriving to that point, did you consider couple's counselling or actively seeking to rekindle that attraction? I'm including this web link, not because it's perfect, but it does have some interesting points on how to approach the loss of your sexual desire for your partner. It also has some links to other sexual resources, such a sex toys, should you ever want to explore that venue.
When you spoke to your partner about the idea of an open relationship, did you also explain your views on love and lust? It is possible that she feels threatened and insecure with your proposition because she is afraid of losing you or because she would feel less adequate if she cannot fulfil your needs. While I commend you for initiating a discussion with your partner on the subject, and even though I'm not sure how long the conversation was, it is something that merits even more conversation, considering you are suggesting a completely different way of approaching your relationship after almost 10 years. The idea of an open relationship is not one that everyone will agree on, and it is best to respect your partner's wishes should she be adamant on the subject. Eventually, you will have to decide if it is fair for you and your partner to remain in a relationship where you are no longer attracted to her, and if you are capable of continuing the relationship if the attraction cannot be rekindled.
Once more, while you have already discussed the issue with your partner, I would recommend even more discussion on the matter. This complex and delicate situation has many options, some stated above, that you could also find on forums for people with the same issue of losing their sexual attraction to their spouse. If you would like some firsthand ideas, you could explore those public forums on the internet with a simple Google search.
I hope these suggestions are helpful and I wish you the best in your endeavours,