I know just how you feel. My cat is 14 and has been acting strange also, not at all like himself. We have taken him to the vet 3 times, they can't find anything healthwise, just that he's getting old and his time is getting near
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I hate thinking about putting him down, but I know there's a possiblilty it will have to be done. As of now, he doesnt seem to be in pain, so I will just let him go in comfort when the time is right.
If you also strongly think that Tigger's not in pain, I think you should let him go comfortably and peacefully when it's time. I know this decision is so hard to make.