I'm on the brink of cheating
Why do you feel that you are on the brink of cheating on your partner?
My boyfriend lost his sex drive when he turned 40.
It's very disrespectful to consider cheating on your boyfriend because of his lack of sex drive because people in relationships are supposed to be loyal. If you have an issue with his sex drive you should try to discuss it with him and try to work something out because that will be more beneficial for both of you.
Sorry but I do what I want.
Well if you talk about your partner's sex drive together you may be able to find a way for him to raise it. There may be something he can take to gain it back instead of you simply leaving him for this reason.