Somewhere around day five, a lot of people stop thinking "I miss them" and start thinking "maybe I made a mistake." The bad parts of the relationship start fading and the good parts get brighter. You replay the best weekend and forget the month of arguments around it.
This is usually your memory doing what it does under stress. It isn't a reliable signal that you should get back together.
Missing them and wanting them back are different
You can miss someone deeply and still know the relationship wasn't working. Missing is about loss. It shows up for things that ended for good reasons too. Wanting them back is a decision, and day five, when you're tired and lonely, isn't a good day to make one.
If you're unsure, give yourself a date. Tell yourself you won't make any decision about the relationship until day 30. That takes the pressure off today without pretending the question doesn't exist.
Go back to what you wrote
This is exactly why writing your reasons down early helps. Read them now, slowly. If you didn't write any, try it today. Think of a specific moment when you felt unhappy, unheard or anxious in the relationship, and describe it in a few sentences. Specific memories hold up better than general ones.
Talk it through with someone
A friend who saw the relationship from the outside can help when your own view is foggy. Ask them what they noticed. You don't have to agree with everything they say, but it helps to hear something other than your own looping thoughts.
Watch the timing of your doubts
Notice when the second-guessing is loudest. For many people it's late at night or after seeing something that reminds them of a good moment. Doubts that only show up at 1am, or after a sad song, are worth treating with some suspicion. Check whether you still feel the same at lunchtime the next day.
If you still feel unsure
That's allowed. Doubt doesn't mean you have to act. Staying in no contact keeps every option open, while breaking it now mostly means making a big choice on a bad day. You can look at it again with a clearer head in a few weeks.