I do sometimes think about "Where would I be if..." If I'd just started sooner... if I never gave up... if I started doing activity sooner, etc, etc. I am one of those people who lost 100 pounds and gained much of it back. But, like you say, the past is the past.
I think the best way to use that type of thinking is in terms of the future. For example, "This time next year, I'm going to look back and be so happy that I started NOW.... not wondering about any what-ifs anymore. I'll already be DOING what I set out to do."
Sometimes I even do a Mega Future outlook: "Ten years from now, I'll likely be at my goal, and long into maintaining. I'll be doing much of what I intended to do. And it's all because I started now, and am working on these healthier habits right this very moment."
Where would I be now, if I hadn't quit?
couldn't even believe it. I was down 15 and now the heaviest in probably 10 years. I said the same thing, where would i be if i hadn't left WW. Then a freind said 'shoulda, woulda, coulda' 
We can do this!1
(And love the pun!)
