When you posted last year I thought it sad you and your DH had got to this point. I wasn't convinced you and he had really got past that affair you had, (though if memory serves wasn't he dallying with someone way before you had your affair?). There's a difference between rebuilding trust and grimly hanging on and as things stood they weren't promising much by way of future happiness.
I remember the reason why you didn't want to split up was mostly to do with keeping stability for your DCs. I am not having a go but do you ever wonder whether the DCs are oblivious to the atmosphere at home?
Cancer is a cruel disease it is not surprising you are buckling under the strain of both parents being ill. If you try and block it out by plunging into work I can understand unfortunately your DH probably feels bad for thinking it but perhaps secretly resents that you are pulled in different directions.
Work has been a sanctuary in spite of your ageist boss who has always sounded a difficult character to work for. If home life were sweeter I don't think you'd pour so much of yourself into work.
I know 28 years is a long time but of that time how long do you reckon you and DH have been happy? The first dozen years perhaps? Married life isn't all daisies I know. If you reflect on your experiences consider the good positive things and then count up unhappy phases = his illness, rows, differing sex drives, sulks, rows, selfishness that slowly emerged (eg you sleeping downstairs on a couch for years because of his snoring, refusal to bother with family holidays yet the golf trip with his buddies), eventually unfaithfulness or porn habits that outweigh intimacy and basic affection.
If I recall correctly it hurt your feelings when your DM said something like enduring to this extent is existing not necessarily living. She was trying to tell you life is short but too precious to cling onto something that no longer sustains you. It is scary looking at drastic change but for one who gives so much your relationship with DH drags you down when in every other area you are coping so much better.