It can be hard at first, the first week will probably be the worst.
You are probably used to messaging him on a regular basis, it's hard just to break that habit. Maybe text other people? Friends, family? On a regular basis every day. Say good morning to your family!
Try to keep busy doing other things, remember how absolutely awfully he has treated you. Have you made a list? Evenings can be hard, try to make a plan for each evening, calling someone, binge watching a show, going somewhere for dinner., go shopping, anything to keep busy for that first little while, even if you don't feel like it, perhaps even a walk if it's not to cold.
If you feel like texting him, ride it out. The feeling will pass. If you are sitting there thinking about him or texting him, get up right away and do something else. Put a face mask on, paint your finger nails, wipe the counter - anything to shift your focus.
After a week or so the habit will be less intrusive. You can do this, it might be hard at first but it will be well worth it. Post here and vent.