Bless your heart, you are having a rough time. Sending a big hug your way.
My opinion is that you can only help people so much and it looks like you have reached that end with the one son. I would explain to him that this situation between him and his girlfriend and his health is taking a toll on your health and you cannot intervene between them anymore, that they need to work on their own problems without dragging you into it. Sometimes to help people stand on their own, you have to make them do it. I am sure it is hard and I wish you all the best and him and his girlfriend too. Also, hope your other son get some good news. Get well soon to all of you!!!