I don’t want to let go

I took my baby girl Chica to the doctors today and they told me something that I had a feeling they were going to say. They told me I should k*ll her. Well put her down which to me is the same thing. I’m so hurt. I’ve had this dog since elementary school. We’ve lived in 3 states together and she means the WORLD to me. I don’t know if I can do it. I’m praying she passes in her sleep because I’m a coward and can’t do it. It’s not right. Is it wrong of me to not want to let her go out like this???? She’s 15 years old and I’m 23. I’ve had her for more than half my life and I don’t know what I’m going to do. She’s more than a dog to me. She’s my family, my first love.