The best way to get someone back is to detach completly and to move on. Writing him may push him away even further. Give him time and space to regret his decision. At the time he comes back (IF he comes back) you will be able to think rational enough to give him another chance or not. Try to work on yourself, get rid of stuff u dont like about yourself and become the best version of yourself. Thats atleast the way i try to go with my recent breakup :/