How do I pretend to be happy when my heart is being shredded?

Today I took my dog to the vet and he is in renal failure. He can have treatment one day a week and it will be very expensive because he is also being treated for heart problems. If I stop the heart meds, his lungs will fill with fluid and he will drown. If he continues on his meds he done. It would just be trading one death for another. He just may last a few more weeks if treated for the renal problem but eventually we will run out of money or his body just won’t be able to take the treatments. So my husband and I have decided to let nature take its course. Which means we will probably have to put him down soon. I don’t know how to be around him without crying every few minutes. How do I make him feel loved and happy when I feel so lost and sad? I want him to feel as well as possible till the end. But I feel like my heart is being squeezed.