You haven't failed by joining once and leaving; how often have you learned how to do something by trying it and learning from your mistakes? Did you play a musical instrument when you were younger, or do you do any crafts? If so, you'll know that you had to practice to before you did things correctly. The old saying of try, try and try again has much merit - particularly when it comes to losing weight! I joined SW twice, both times online. The first time I couldn't get my head around the plan and effectively fought it - it seemed daft to me that I could have unlimit pasta/potatoes etc, but only a measured amount of bread. Also, I made the mistake of not reading the Superfree lists correctly, so I was having peas and sweetcorn which are not Superfree.
However, I saw the result of doing SW correctly in my niece, so I joined for the second time and this time I decided to trust the plan - and to bear in mind that potatoes etc are limited by the 'eat until full, not until stuffed' rule. I've lost just over 4 stone and have maintained for well over 6 months. You can have the odd blip/meal/day/week off plan, but if you get back on and persevere you will reach target.