Thank you so very much to the 12 kind people who left such lovely messages in the comments. I've read every single one several times over, and have felt comforted.
John is declining faster than any of us thought, and we will probably be moving him to St Peter's Hospice in the next day or two, where they will know how to keep him comfortable as he nears the end. There I shall be able to visit him whenever and for as long as I want (hospital visiting hours are so restricting). I had wanted to care for him at home for as long as I could, but he quickly became too weak and bedridden, and he's too heavy for me to manage on my own. His intermittent mental confusion also means that he wants to move around frequently, and the district nurses who help people who are convalescing at home can't be expected to be there 24 hours a day.
Today during my four-hour visit it was the first time he did not acknowledge that he knew who I was, and I feel as lonely as it's possible to be. I have in effect already lost him. May he not tarry to feel any more pain, discomfort and helplessness.