I can’t lose weight.

I’ve put on a lot of weight over the past two years, and each attempt to take the weight off seems to make things worse. The problem is that I get frustrated by how slowly I’m losing, and I give up. For the past week I ate nothing but 1 plain potato and 1 spinach and chicken salad each day and I gained three pounds. It’s probably water weight or something along those lines, but the truth is that it’s just much, much harder for me to lose weight than it used to be and I have a really hard time sticking it out when I don’t see any progress. Suggestions? (I did just start on thyroid meds, btw, but they don’t seem to have helped so far).