I'm so sorry
I just layed my first baby to rest in Janurary. He was almost 15 (his birthday is in two days) and I raised him from 10 weeks old. The best advice I can give you is this: it is going to hurt, there is no way it won't. But it is the most loving thing you can do for your baby, to set them free from the pain and to be there with them when you do. They can not make this decision themselves, we must find it in our hearts and be in tune with them enough to know, when the pain is more than they deserve to go through.
I won't lie and say it is easy, and I am sure it is different for everyone. For me personally it was one of the hardest moments in my life. But as time has gone on, and the happy memories replace that day in my mind, I realize it is more about my own selfishness, that hurt and pain and grieving was about me being unable to let go. But I know now my baby is in peace and the best choice, as hard as it was for me at the time, was for me to be there with him and hold him in my arms to feel my love as he took his last peaceful breath.
Bless you and your Chloe