Planning is really important, especially if you don't have a structured schedule. If you eat out a lot on the weekend, start by changing where you go so that you have healthy options to choose from.
Or start new habits like creating fun healthy dishes to eat at home - trying new recipes and invite your friends over to try them (but don't call them healthy LOL).
Bring certain portable snacks with you if you are out all day. Things like string cheese, zone or balance bars, zip bag with some nuts or dry cereal. Something that you can pull out and nosh on knowing exactly what you are eating instead of giving into temptation somewhere.
The more you practice this on weekends, the more it will become your habits and it will get easier over time. Not easy, but easier
I have those times where the best laid plans end up on the floor all trampled on - but I know that the very next meal is an opportunity to get back on track.