Start with calm, not defense
The first text after an argument should lower the temperature, not restart the debate. Focus on the relationship and the immediate goal of reopening communication.
Take responsibility for your part
Even one honest sentence of accountability can make the rest of the message easier to receive. You do not have to solve everything in a single text, but you should avoid sounding dismissive.
Keep the door open
A useful closing line invites conversation instead of forcing resolution on the spot. That gives the other person room to respond when they are ready.