If you're already doing the exercise but not losing weight, there's not much else of a choice but to alter your food. And your diet is not that unhealthy in terms of WHAT you're eating, then the only thing left to go for is quantity.
As for going back to eating the same as you did before - this is fine, assuming you were maintaining a constant weight before. If you're 10kg overweight and have been for a long time, then you drop the 10kg and resume your previous diet, you should continue to have a static weight at that point as well.
But if you've been gaining weight for years, then go on a diet and then go back on the same diet that was causing weight gain, that does seem a bit of a no-brainer.