I have to put my four year old cat down today, how do I cope?

He has liver failure, it's congenital, which means he's had it since he was a baby. We rescued each other and my mom keeps telling me he was a gift, which he was. I love him with everything I am and my heart is breaking. I've never done this before. He was such a fragile baby and I worked so hard to get him healthy, but now he is so thin and so tired all the time. I don't want him to get worse. He's not in pain, the vet told me that people and animals with liver failure just feel ill, and he's still so very loving, and occasionally he's very perky too. My mom and I are struggling, we're both going to miss him tonnes and tonnes, he's really changed our lives? How do I feel happy again when he's taking my heart with him?