Hey phr4n, I was in the same boat - almost to the lb. I went from ~16stone at 16 years old, to a ground quaking 22stone at 21. Depression was also a huge part of it, for various reasons.
The path I took is not recommended by medical experts, nor in fact by general consensus, but I felt I had to do something, and anything would be 'better' than the state I was in. So I bought the Atkins book. 5 months later I was 4.5 stone lighter. I lost more after that, simply because of the next point.
Most importantly, my appreciation for my body and my food is back. I am constantly keeping an eye out for what I am putting into my body - and not by determination; it's happening 'naturally' in that it is second nature to me, I don't have to keep reminding myself to check etc.
80%+ of fat loss is down to diet. Unless you are some miracle athlete like Michael Phelps, you simply will not lose fat from exercising alone.
HOWEVER, I do regret deeply not exercising whilst dieting. Not because I would have got more fat loss, but because I was a complete wreck of a man when all that fat was lost. It took me a year of exercise to feel normal, whereas I am certain that had I started when I started the diet, I'd not have been the wreck I was.