I used to hold in my stomach as far as it could go from aged 10-15 years every time i wasnt alone. could this be responsible for the chronic constipation i experience? could it cause organ damage?

Hard to guess: Each person with constipation needs to be sure there is no other disease causing it. If constipation is just due to slow movement of the food and poop through the gut, then the things to try are vegetables, water, prune juice, prunes, fiber plus water, Miralax (polyethylene glycol) Senokot or other senna, milk of magnesia, etc... A doctor can evaluate, and might prescribe Amitiza or Linzess.