Well, frankly, I had to go off the med that made me gain weight and try a new med.
Yeah, exercise helps with depression. If you are too depressed one day, try at least a five-minute walk. Take advantage of the good days and get a lot done then. No, that's not the regular workout that we're supposed to be getting, but it's better than nothing!
I've always been able to picture my goal weight, but I can kind of relate to you in that I never thought I could lose the weight. You will notice a difference, even if the difference is just in how your clothes fit. So even if visually you have a hard time picturing yourself as thinner, if you use an objective measure, like clothing, you know you've had some success.
Good luck!