We were to get married, but he left me. I can't explain how distraught I am. I can't focus on anything I can't sleep or eat properly, and I don't think I will ever be the same. I don't want him back in my life. How do I move on from this?

Time heals all. Youre not the first one to have this happen to and definitely wont be the last. Actually , as bad as this sounds, youre fortunate to have this happen now instead of problems and possible divorce once you got married to him, so with each passing day, the pain will sibside a little more and soon youll move on again. Try not to think so much about what you lost but what you gained here. You actually got rid of someone who obviously really wasn't very much into you here. I am sorry to hear this but life will go on now and so will you.  Good luck