Hang in there!
Don't give up. I have been riding a roller coaster just like you. It seems as though every time I start to significantly change the way I look, I start sliding back to old habits. At the end of my rope last week, I went onto amazon and bought the book, "Make Shift Happen." by Wayne Dyer. I am finding it very inspirational and I am also finding some great tools and tips to change my thinking. Maybe this is something that would help you? I am not a huge fan of the website, but I am really enjoying the book.
Other than that, just try to make better choices one at a time. When you reach for a soda or start heading to the drive through, you could start asking yourself, "Is this REALLY what I want to do right now? Could I make a better choice now, and perhaps come back to this snack later?" Starting to make the right decisions more often than not has to be better than just giving up..
I hope some of this helps. Remember, many of us are right there with ya!