sorry man. it's so incredibly hard but you're doing the only thing you can do. an 18 year old lab is amazing. i have a 13 and a half year old lab, and he seems to be having more trouble every day. i've started doing the mental gymnastics about what to do when his time comes, but i'm bracing for it. you've been that dog's best friend for 18 years, so keep up with that mentality and give him a peaceful send off.