I have done that so many times! I feel your pain!!! Luckily I usually knit things for my kids, and the biggest is 3 yrs, so it's not much knitting no matter what. But, it's completely depressing when you finish and realize that your gauge was off (and you would have known if you would have bothered to measure) and now the 3 yr old sweater will have to be saved until he's 5.
I just laugh and move on, my son will have some amazing sweaters when he gets a little older! And me, well, i have learned my lesson.
Good luck on your sweater!!!!