Hi thin_thin, I can relate to what you're saying. I was always a chunky kid and probably gained and lost the same 15-20 pounds over and over and over again beginning in high school. For the most part I was about 127-130 but the seesaw went up and down with marriage, pregnancy, grad school, etc. I am the same height as you, 5'2", and I think I was probably about 145-150 at my highest weight, though it was always changing. I would lose the most when stressed, but that's not a good way to go.
Currently I am at my lowest adult weight. I have only maintained the loss (of about 19 lbs this time) for a month and a half, so I'm definitely no expert, but using FitDay will help you tremendously. It has definitely taken me to a lower weight than I ever achieved before, and in a healthier way. You will learn so much about how to structure your eating and exercise to work for you; it's all there in black and white, and the support here is phenomenal. Jump onto the boards and ask for what you need and make yourself accountable.
There is a subgroup for people wanting to lose about 50 pounds, I think; also one for 5-15, 30, and 100...you could hop into any of those (we have someone at 5-15 who has more to lose but that is her first goal) and get to know the ladies and get awesome support.
Good luck; you can do it!!!