We had to amputate our golden retriever's front leg because of a fast growing tumor - and she recovered in record time - was up and walking the same day, running around within 2 days! She was very happy and normal after the amputation and lived (very happily) for another 5 years. I wouldn't hesitate to do it again (if necessary of course!)
This was about 10 years ago - but really, she did absolutely fine with it. She was 9 when we had to amputate the leg. There was no choice, like in your case, except to let her suffer. She was very sensitive to touch there (obviously) for few weeks.
I know it's a hard decision, but for us, we wanted to give her a chance - we figured if it didn't go well, we were back where we started more or less and at least we gave her a chance. Thankfully, it turned out well - I'm sure it will for you, too!
Kim