Pollyj - I am sorry that this is a difficulty for you at present.
I absolutely recognise that you have no sexual desire at present and that this is currently a mismatch with your OH. This does not mean that your marriage is on the rocks, or that you do not have a sound relationship. It simply means that there is a problem that you both need to find some way to solve.
There is nothing wrong with you - you are simply at an age when your hormone levels are such that your desire for sex is low or non-existent - you have done nothing wrong, and are not ill. There is no fault on either side here - it is just an unfortunate mismatch at this point in your lives.
I faced this problem with my OH (who has since died) and fully understand how hard it is and indeed how destructive it can be. It becomes impossible to show any physical affection - a hand-hold, a hug, an arm round the shoulder - without this being seen as an invitation to sex. That is very distancing. In our case it was compounded by the drugs he was taking for his PD, which are known to cause increased libido in some people. It became an obsession with him and some of the social niceties that surround a sexual relationship went out of the window from his side. I was under siege basically and had no peace from his advances. In addition, because of his illness, it was not possible to have a rational discussion with him and find some happy middle way, as would have happened had he been his former self. This was all a very difficult situation and I look back on it with much sadness. He was far from himself, and the challenge of dealing with this was enormous.
But hopefully, you are not faced with this extreme, so there is hope!
I really do think that the only way forward is for you both to set aside some time to talk about this together. Time when you are not locked in an advance/reject situation in bed. Hear his side of it; how it makes him feel when he is "rejected"; how much he understands about your side of the equation; what exactly he is asking for; whether he is able to sustain affectionate gestures from you without assuming it is a "come-on"; whether a snuggle-up in bed equates with sex for him; or can he do this without wanting more; what he might see as solutions. Then he needs to listen to you and to understand how it all makes you feel, and what exactly you do want (rather than simply do not want).
So, a starting point would be to think very hard about what you do want from him in terms of physical affection; and what exactly you feel able to give.
You need to finish up in a situation where your OH does not feel a constant sense of frustration and rejection; but where you do not feel you are being asked to do something you do not want to do at any given moment.
You need to think about about what you are able or willing to do that might relieve his frustration, and satisfy both your needs for your close relationship to continue on all fronts.
He needs to be honest - "It would help me if you could do X"; and you need to be honest about what you feel able to do.
I really do feel for you - it is so very hard when, in the context of a loving relationship with decades of shared memories and achievements, there comes a moment when things get out of kilter through no fault or either person.
You might be able to agree a period when sex is off the cards, but physical affection is not. This will give you a breathing space to gradually move towards a solution. The solution does absolutely not need to be about you "giving in" and finding yourself doing something that you do not wish to do - and I am sure he would not want this either.
There will be a way forward I am sure; but the most important thing is not to le this pull you both apart; to listen to each other with a true willingness to understand; and to totally avoid any "blame language."
You do not need counselling or medical treatments - you need communication and a willingness to try and look at this in the same way as all the other problems that you will have had to solve during your marriage. A joint approach with respect on either side.
I truly wish you lots of good luck with this shared challenge and hope you find the solution that is right for you both.