I am so, so sorry for your lost. But you cant give up, that would be a betrayal. Its always heart wrenching and there is nothing I could write that can make that go away. My old girl died last Fall and it was also very sudden. What you need to remember is that I am sure you gave him the best life possible and because of you, he had an happy, carefree life. Today, you have to deal with all the pain, but in time, all the good memories will come back and you'll see that the grief is outweighed by all those years of companionship. Most importantly, your capacity to love is still intact, and in time, you'll make another dog happy. That is the best testimony you can give to your dog, that the pain you're going through right now does not overshadow all that he gave you, all that he meant for you. You cat is counting on you to hold on. And so are many others you do not know yet. There are so many pets, each one of them with their own personality, with their own qualities that so desperately need and deserve a good home. But for now, feel what you have to feel, knowing that broken heart can be mended. One thing that helped me a lot is writing a memorial online for my girl, Lenore. So you might want to try this. Here is the link:
http://www.dogquotations.com/write-a-memorial.html
It helped me a lot to realize I was not alone with those almost unbearable feelings of loss. It might help you too. There is also a great book, Letters to Strongheart by John Allen Boone which helped me a lot going through loosing her. Here is a vid with a reading from the book, which may bring you some solace:
http://www.youtube.com/watch?v=vU_1mtf42_0
I doubt very much it was the flea meds you used. And if it was, you had only the best intentions, so dont blame yourself. We all try to do our best for those we love, and sometimes, it is just not enough or we make mistakes. This does not alter the spirit behind our intentions. You dog knew this, knew you love him, and in the end, that is all that matters. Take care.