How to get back onto a healthy and successful diet?

Based on all the information you have given I would suggest seeing your family doctor and having your thyroid tested. You are eating at a DANGEROUSLY low calorie count. Your muscles are going to start to atrophy, your general health is going to suffer and by restricting your calorie count at that level you are causing your metabolism to bottom out, making it even harder to lose weight. Your next step should be a doctors appointment.