Well I think you need to try to educate yourself about food and diet, try to learn things like 1 chocolate bar today won't make me/you/us fatter, but having 1 today 1 tomorrow 1 the day after might just do. It's pretty much the same for any of the foods we eat, too much too often and we will obviously gain weight but having smaller amounts or unhealthy options less often is certainly the way to go.
Also don't be afraid of eating too many fruits/vegetables, you would need an insane amount of these (unless they were all potatoes) to actually gain weight. Try to have a good relationship with food and have a little something of what you like every now and then even if it's unhealthy - only by actually having it and not seeing your weight increase will you learn to trust in what I am telling you.
Good luck with overcoming your issues.