I find that honestly what helps me the most is to be kind to myself. That means that I am no longer allowed to beat myself up when I go off plan; instead, I try to learn from the experience and to know that I will make a better choice next time. Try not to be too hard on yourself; when I really get down on myself, is when I choose to give up and fail. Instead, I tell myself, "You could have made better choices today, so now it's time to let it go and move forward." Otherwise, I just get into a spiral of negativity and fall right back into my old habits.
I'm slowly learning that even though I might *think* I want that second serving of ice cream or I might really WANT that donut that's on the break room table, I will not feel good about it after I eat it so it's better to just not. It's tough, I know--old habits die hard. So sometimes I do fail, but what matters is that I don't let that momentary lapse become more than a momentary lapse. I didn't get fat from eating one donut, but if I keep on doing it I sure will!
Hang in there. Learn from the experience. As Judipurple suggests, maybe this means you need to tweak your plan so that it eventually becomes something that's not too difficult to stick with.
You can do this! You are correct when you say this journey is NOT impossible! One last thing that helps me is to go read in the Maintainers forum. It also helps to look at the "Before and After" pics, to realize that people who had an even higher mountain to climb than I did, managed to make it so maybe I can, too!
One dancer for each 5 lbs lost!