So Sorry Nick, I know exactly what you are going threw....life's a real bitch sometimes.
Sorry for your loss Nick . . . I know that road all too well as I had to travel the same road 4 years ago when my 16 1/2 year old Yellow Lab "Belle" ( she was named by my daughter's after Belle from Beauty and the Beast that had just come out ). Got to be one the hardest decisions that I've ever had to make - but Winter was approaching, and she was not doing very well . . . so it was my time to be the "responsible pet owner" and relieve her of all her pains and not let her suffer anymore. Thought that I would do fine . . . boy was I wrong . . . it tore me up so much as she had been with me through a loss of a job, loss of a marriage, and was my best friend - her tail always wagging - when I came home from work to an empty house as my (2) daughters were living with their mother 150 miles away.
The guys at my office understood . . . they left me alone, and it took me a week before I could talk to anyone without breaking down . . . they're part of our lives ( I'll always miss her ).
It's a very hard decision - but you're taking care of your best friend, that has been by your side for all these years taking care of you ( and you didn't even know it ).
Rest in Peace Maxwell . . . say hello and go play with Belle - she's a great gal . . . and all my condolences to you and your family for your loss.