Weight loss problem! Please give advice!?

Ok, here's the story. I am not obese just a bit overweight. I am on medication that retains weight but it shouldn't be keeping me from losing the 15 lbs I'd like to lose. I've been trying since January to lose this weight. I've gone to a gym 4 times a week, I walk an hour every day, I do strength training, I've maintained a healthy diet, and I've had a personal trainer. I lost 3 lbs and that was it! I did lose in inches but it wasn't a whole lot. I finally became frustrated with constant working out (plus I got pink eye 3x from my gym) so I quit the gym and relied just on my own walking and some strength training. Once I stopped trying I lost 5 lbs! But the joy was short lived as I gained 4 lbs when I haven't changed anything! I'm so frustrated from my year of hard work with no results and now I'm scared of the quick approaching holidays! Anyone with any advice PLEASE help!!!!