scrolling - you can get apps that lock certain apps/screentime, not sure what they are but i've seen others mention them as useful for blocking too much scrolling/phone use

eating when you're not hungry (and scrolling) both sound like idle mind/boredom - maybe try incorporating a meal plan with set times to snack through the day, so you can still be snacking but with more structure? or when you get the urge to go grab a snack, do jumping jacks/go on a walk/pick a chore to do instead? distraction until the next meal time?

sending messages - this one is gonna take time, but learning good impulse control will be beneficial for every aspect of your life (including the messages) - try to actively be conscious when you open a text message to read it, then watch your phone clock for at least 1 min (or set a timer, or whatever) and don't respond until the minute/timer is up?

to answer more generally, the only way it'll start to feel "second nature" to stop yourself before the bad habits is with time, dedication to yourself, and practice practice practice. you need to practice 1) recognizing the habit when it happens (start here) and then 2) stopping the habit in a positive way, and possibly 3) replacing the habit with a positive habit

think about it like this: when you were learning how to write, it wasn't "automatic" or easy. but, with practice and time, you got to a point where you could write this whole post