What to do if i had a hypoglycemic seizure when my blood sugar hit 31. can this cause lasting damage?

Yes: Your brain requires glucose to function. When sugars get so low that you have a seizure, this can be associated with brain cell loss. Diabetics can suffer brain death if sugars remain low for too long. Avoiding severe hypoglycemia is much more important than avoiding high sugars. If you are having frequent episodes low sugars (<70), your medication needs to be adjusted.