I have a 15 year old mixed breed dog. I think the time is coming when I'll have to put her down. She has had arthritis for years (has been taking Rimadyl) but now her back hips will just give way sometimes and has a difficult time of getting up. She's blind in one eye due to a tumor, and sometimes looses control of her bladder. Is it time? She is my first dog (had her since she was 3 montbs). How do you know?