hey! I am also currently around 120 lbs and looking to get down to 100 lbs (but I'm a good 6 inches shorter than you). I have been at that weight almost my whole life, its where my body is comfortable, and only recently have gained 20 lbs in the past 5 months due to stress.
My plan as of right now is to just eat less, hard for me because im prone to snacking...even when im not hungry. i started a bet with someone in my family to see who couldnt eat chocolate the longest as a little competitive inscentive to keep me going.
and i also started swimming 6 days a week.
my two pieces of advice are:
1 turn it into something fun, like i did with the bet. it keeps you going when youve got someone else holds you accountable and joins in. plus it makes it seem more like a game than a task
2. let it come with time. at first when i started gaining weight i would say stuff like ill drop 5 lbs in a week and itll be gone, but now i know that the only way a healthy life style change can happen is if you give it time
since we are both currently looking towards the same goal weight, if you ever have any questions or anything im here!
i do have one question for you. About how many calories are you trying to eat a day? right now i dont have any set plan, i just try not to snack between meals, but i still think my calorie count is pretty high.