"Weight loss is 80% nutrition... and you cannot out-train a bad diet."
I would add that your diet doesn't need to be "bad" per se to hinder weight loss -- you may be eating all the right things, but have not cut your calories enough to create a deficit.
I lost a lot of weight about 10 years ago, but hit a really intractable plateau, that I just could not seem to bust through no matter what I did. I still have my food journals from that era, and I now realize that even though I was eating well, I was just taking in too many calories to lose any more weight. Not tons of junk food, mind you -- it was mostly pretty healthy stuff (or at least semi-healthy stuff like flax waffles), but too many calories overall.
It really does get harder the closer you get to goal to both keep your calories low enough to lose weight and yet enough to still have good nutrition. -- especially for women. I've managed to do it this time, but for the last 30 pounds or so I really had to pay close attention to my food intake (both calories and nutritional content). Exercise really does make that easier, but I've found that excessive amounts of exercise can actually make weight loss harder -- because you get so hungry that it gets very difficult to maintain control of your diet. Of course, what constitutes "excessive" exercise varies from person to person... for me, I just tried to make sure that my appetite wasn't going haywire. If I was starting to get too hungry, I pulled back a little.