Making a Halfling Warlock but don't know what backstory should be

I'm building a Stout halfling Great Old One Warlock for a DnD campaign. My DM uses the character's backstories in his campaign and I don't know what my PC's backstory should be or how to make up compelling NPC's, artifacts, memories, etc. that can tie into the gameplay in a useful way.

Things I want to incorporate in the backstory:

  1. He's Canadian (or the Canadian equivalent for Toril).
  2. He is very socially anxious and has a lot of PTSD.
  3. He is made fun of for being chubby (although his weight is normal for a halfling).
  4. He is middle-school age.
  5. Although a warlock, he is quite skilled with a cittern.
  6. Very intelligent and very un-charismatic for a warlock (-1 charisma).
  7. I want his patron to be antagonistic toward him (maybe it possesses him or smth)

Every comment counts!! Thank you all so much.