Oh man, I can't believe you reinstalled for something that was fixable! I wish I'd seen this thread last month...
Alas, there's two similarly named groups: adm and admin. You should have added yourself to the admin group and then logged out.adm is for internal use only! Do not add yourself to that group!
(Yes, I know it's messed up that there's two similarly named groups on the same system. I used to get confused.)
Confirm what I said by running groups under your reinstalled system. You should see an instance of admin, and probably not an instance of adm.
OP's account was the only one (as stated) so he would already be in the sudo group (that's default); it was not necessary to add again to sudo group. From what I read and understood, there was no issue but only a misunderstanding about how sudo works. Comments #14 and #18 have explained clearly.