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I had to do the same with my previous dog Pinki- named Boo (and where my name's from).
I had him for 8 years and he really was the baby of the household. Sadly for him, Mini Boo came along and whilst we tried our best to make him still feel special, he hated her, which took an awful turn when she was about 2 or 3 and he bit her on her face.
I was going to have him put down, but couldn't bring myself to do it- luckily some friend of my mum took him on, but i still miss him.