You could also do what many, including me, had to do and be a bit sneaky about it! Introduce a "person who's new in the neighbourhood - nice for them to get to know someone and make friends" - introduce as a friend and gradually get them helping? I brought a man in "to help in the garden - to do the heavy work".
Of course, there comes a time when we have to realise that when dealing with someone at a certain stage of dementia, we cannot just accept what they say in things like this - their reasoning and comprehension is affected by the dementia, and your OH will be unaware that in fact the help is needed, and, often, the illness makes the person actually not care whether help is needed or not - they can only see what they want and the illness will not allow them to see (or care) how you are affected. I know that sounds harsh - it's not meant to. It's just the way this awful illness affects people at particular stages. I found that later, my husband would do anything if he thought it would make things easier for me, or if he thought I was tired or ill, he would be dreadfully upset! So, what I'm saying is that if you both need the help, then you can't let your wife decide whether or not you have help in. It might take time for her to adjust to having someone coming. But things won't get better. The need for help won't go away. Chuggalugg is right - start with the GP, and try and get a Carer's Assessment. I'm not sure of what else is involved over there. But as all these things take time, if you are even thinking about it, then it's probably time to put the wheels in motion.