Good job on the 15 pounds !!!
Keep a daily log of intake and count the calories you ingest on average each day.
Figure out the calories in, minus the calories burned.
Whatever you have left, you should eliminate 3500 more calories from that total [per day].
Then either burn an extra 3500 calories or take in 3500 calories less [or a combination of both]
3500 calories = about 1 pound
It sounds tough but if you are working full time it really isn't ... you just have 5-10 minutes here and there during the day to update the log.
I always have to remember that it took me way more than 3 months to put my weight on, so it is going to take way more than 3 months to get it off.
Congratulations and good luck