I just went thru this with Yana, my 15 year old best friend. He was a German Shepard / Boxer mix. I had him since he was a puppy when I picked him up from the pound.
He was the same, wanted to walk every day but he started where he would walk down hill but couldn't really walk back up. About a month before he had a seizure in the middle of the night but seemed to mostly recover. Since then he had lost a lot of weight and was having trouble standing with his back legs.
I could see it in his face so on Memorial day I made the call and took him to the vet. At least this way he went peacefully in his sleep and I could have a clear continence knowing I wasn't selfishly keeping him going too long. I was anxious while taking him there but felt so relieved after it was done.
RIP Yana
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"I hope yours took a bath once in a while, kid. This guy's armor smells like he lived in it."
―Han Solo to Luke Skywalker, while donning TK-422's armor.