Oh I do understand your dilemma. Several years ago my OH was diagnosed with Parkinsons Disease. We already knew he had it many years before that, but delayed a formal diagnosis until my OH (a retired doctor) decided that the point had come when there might be some benefit in treating it.
So....who to tell and how? OH wanted to tell no-one, but people must have noticed things weren't right and we could not just ignore it. And I felt very uncomfortable with people we knew well just pretending all was fine. But he had the right to decide how he wanted to deal with it. As a well-k own and respected local professional he found it very hard to acknowledge weakness.
I encouraged him to be honest with people about the fact that he has a long term degenerative illness and strongly opposed him keeping it from our children and close family - it felt so dishonest and unfair on them - and on me in a way, as I felt that we were living a lie with our dear children and I could not cope with that.
We did tell them and they were sad but of course hugely supportive - they treat him with great respect and value hos talents; while at the same time making allowances for his problems.
I encouraged him to gradually tell friends and everyone has been so good - both to him and to me.
It is so much better with it all out in the open - no-one makes a big deal of it, but everyone makes allowances when he forgets things or falls asleep in the middle of a conversation or is unable to write. And they quietly get on with help with heavier things that neither of us can manage.
Living a lie is agony - so very stressful. So I would encourage you to let people know that he has a problem with his memory. The difficulty for you of course is how or whether to engage your OH in the decision to do this. I would encourage you to be open about it, both in his presence and not, as that will take a huge stress off your shoulders. Only you can decide whether he might be averse to your doing this and how you might approach it with him and with family and friends.
I do understand that tricky balance between doing what feels to be right whilst at the same time maintaining his self-respect and trying to do what he might wish.
You have a difficult road ahead of you and I would encourage you to be as open as you can so that all the support and love of your family and friends can help to carry you through. And please do grab any help you are offered with both hands. 
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