welcome to the forum.
It sounds like you're planning to make some major changes to your diet all at once, the thing you do when you go on a "diet".
Making dramatic changes rarely works and the majority of us who do make them "fall off the wagon" as you say.
Making minor changes that become new habits is the key to long term weight loss success.
For example swapping the foods we eat regularly for lower cal options is a good strategy because it means we can continue to eat the foods we like.
Lately I've been simply trying to get more vegetables into my diet.
I've started ordering vegetarian pizzas instead of super supremes and have started eating vegetarian lasagna instead of meat lasagna and eating roasted vegetables for lunch occasionally and I'm happy with my progress.
Because I'm actually beginning to enjoy these foods it's no longer a case of forcing them into my diet and I'm starting to eat more vegetables without even thinking about it.
Perhaps you might try starting off with more modest changes to your diet and then you shouldn't need to nominate a starting date which obviously is something you're trying to avoid.
Hope this helps, please let us know what you decide to do and how it works for you.