I know exactly how you feel. I had to do this in 2005 for my first two beloved boxer babies. It is one of the hardest things we will ever do, to help them leave us when we know it's time to let them go. At first it seems that nothing will be right again, but focusing on all the good times helps to ease the pain and as time goes by the memories become sweeter, happier and less tinged with grief.
My heart goes out to you and your family as you grieve for Killian. It sounds like he was an amazing boxer boy who embodied all the reasons we fall in love with this breed over and over again.
We'll light a candle for Killian tonight and rejoice in all the joyous years you spent together. Godspeed, sweet boy.