Ask people who have done no contact which early day was hardest and a lot of them say day three. The novelty has gone, you've had two nights of thinking, and your brain starts making a very convincing case for one small message.
The case usually sounds reasonable. Maybe they're hurting too. Maybe silence looks cold. Maybe you were too harsh at the end. These thoughts feel like new information, but most of the time they're the same feelings wearing a more polite outfit.
Things that help on day 3
Read your reasons first thing in the morning, before the arguing starts. It's easier to hold a line you've just looked at.
Get out of the house for a few hours. A change of scene breaks the loop of sitting with your phone. It can be something small like a coffee somewhere new or a long errand.
Notice what you're actually missing. Sometimes it's them. Often it's having someone to tell things to, or the feeling of being chosen. Those needs are real, and you can meet some of them elsewhere, through a friend, a sibling or even writing it down.
If you're tempted to "just see how they are"
Ask what you'd do with the answer. If they're fine, it might hurt. If they're sad, you'll want to fix it. Either way you're back in the middle of it, and day three becomes day zero.
If you slip
A lot of people slip around here. It doesn't erase the two days you managed. Skip the second message, don't wait up for a reply, and start again tomorrow with what you learned about your weak moment.
One more thing that helps on day three is telling someone the exact moment you're struggling, rather than waiting until it's over. A message like "I really want to text them right now" to a friend takes the secrecy out of it, and urges tend to lose some of their pull once you say them out loud.
If you make it through today, you've got past a point where many people give up. Day four tends to feel a little less sharp.