When you go on a vacation, it is inevitable that if you are driving, you are going to run into problems. In fact, no one gets into their vehicle thinking, "I am going to have perfect travels, and I'm certainly not going to hit a single pothole on the way there."
And yet, when it comes to weight loss, a very common line of thinking is: This is going to be easy. Because I'm determined. And I'm going to make it happen. And there is absolutely no way I am going to do anything stupid because I'm committed this time. I know I failed the other times, but I just wasn't as focused as I am now. Everything will be perfect, and I won't accept things any other way.
It is this line of thinking that destroys our success more quickly than anything else. A goal of perfection is quickly and easily broken. And once that sense of discouragement sets in, it's almost impossible to gain momentum again.
When setting goals, it's great to set high goals, BIG goals, and plan to work hard to achieve those goals. However, it would be foolish to not plan for the bumpy road that is ahead of us. When planning and beginning a weight loss journey, it's important to plan for the failures; because the failures are going to happen. I'm not talking about overall failure - I'm talking about when you cave and have 2 slices of cake instead of one. Or you feel lazy and decide that you just aren't feeling the workout today, so you lounge on the sofa instead. Or you stay up very late, and competely destroy your sleep schedule you've been working so hard to establish.
If you plan for those failures, and any other minor bumps in the road that you may encounter, you can simply go on a detour rather than getting entirely lost and derailing the entire journey. Take a moment to sit down and make out a mental list of all the things that have interfered with previous attempts, or that you know may interfere with this attempt. And come up with a set of logical solutions (eg. If I eat an extra slice of cake, I will do an extra set of cardio to offset those additional calories).
Don't let the potholes and roadblocks on your journey cause you to get lost and have to turn back. Plan for them, and set up detours so you are prepared to continue on course.