I have gained a little weight and believe that I have basically slowed down or ruined metabolism

. I believe it's due to stress and not eating enough. Somehow I have it in head thinking that eating around 1200 calories and working out for an hour to two hours will help me lose weight. But now, since I gained some weight, I am afraid to increase calories because I think I'm going to gain even more weight.
question is, how can I get metabolism back on track so it starts burning calories faster again? Since Feb. I gained about 8-9 lbs and went from 120 to 129ish

. I can still fit into most of the same clothes that I wore at 120lbs (just not as comfortably or confidently). During feb-midmay I rarely worked out due to school and I def. know I lost muscle during that time frame and gained more fat then I want. Since starting to workout again, I think I gained a tiny bit of muscle back even though the scale has not really gone down. How do I know this? because I can get butt into a pair of jeans that I could in June.
Anyways, I don't want to just wake up tomorrow and start eating 1500-1600 calories because I feel as if I'm going to gain weight and I don't want to do that. Right now I am eating about 1200 calories which I know is most likely too low for the amount of exercise I do and instead of losing the weight I think I did the opposite and put it on by restricting calories so much. what do you think?

But, how should I approach all of this? Should I gradually increase the amount of calories I eat each week? Bossman or swanso what do you suggest I do?