Hi Shellie,
I too have recently had to go thru this same nightmare - my 10 year boxer got cancer and after two very expensive surgeries - the darn cancer kept coming back so we too had to make a decision.
I realized that we were keeping her alive for "us" not her. In the end, we took her to be euthanized. It was the hardest thing to do - this dog was my life - just like a child to me - I was devastated and its now been almost 8 months and just thinking about her will bring tears to my eyes.
I do take comfort in knowing that she is in peace now - no more pain, surgeries, medications, etc...she can rest in peace. I was with her til her last breath and I can honestly say - if you trust your vet - the euthanasia procedure is very peaceful and painless.
Be strong for your dog and do what you think is needed. Best of luck to you while you go thru this hard time.