Our cat is 18. He suffers from hyperthyroidism which he is on medication for, and probable underlying kidney trouble, and has lost half his body weight. He can no longer jump and his balance has been affected for some time now, so he will stumble and fall over regularly - vet is unsure of cause and we've opted not to have any invasive tests. He has daily urine accidents in the house and sometimes poo, but not so much. He has stopped grooming so I do it for him. Due to incontinence in his sleep, his fur gets soaked in urine and discoloured. A month ago he had diarrhoea in his bed and just lay in it until we sorted him out 
He still eats enthusiastically. He is not in any pain, so far as I can tell. DH thinks we should have him PTS. I have said no, then yes, now I just don't know. We are seeing the vet and will abide by their judgment, but I just feel so guilty as I love him and I also know our lives would be easier without him.
Sorry for long post. I keep crying about this, and I feel like an idiot. Anyone else been through similar? Thanks.