It's painful, isn't it?
My DS1 is naturally lazy. We tried 'everything', threats, chats. sanctions, rewards, for limited success.
The school were on-side, seeing a bright but lazy boy, too, lots of mentoring etc.
Our relationship began to break down over GCSE revision, til eventually, I had to say that I was going to stop nagging, begging etc as it was upsetting me and falling on deaf ears; tho I wouldn't be able to stop myself saying, as he banged away on his PC, gaming "You and I both know what you should be doing" as I walked past (cue his dramatic eyeroll, sometimes accompanied by some light-weight vitriol etc).
Anyway, he got one grade less than predicted in 6/10 GCSEs (getting AABBBBBBCC); he drifted into Sixth Form, doing 4 ASs, plucked out of thin air, Maths
, Physics, Geography, Economics, openly lying about 'wanting to become an engineer'.
By Feb half term, it was clear where this was heading, as I think he also was beginning to appreciate; so I told him that a couple of low grade ASs weren't worth the paper they were written on; I wasn't going to support his laziness any more that he either bucked up immediately or he was getting a job at the end of Y12, when he'd be 17. I said I didn't care what but it would be full time and he'd be paying me rent. I absolutely meant it.
Anyway, during this time, I discovered a lad at my work who had done just the same, but he restarted sixth form doing a 3 A level equivalent BTEC instead, thence onto uni. I researched this, and with DS getting an entirely predictable D and an E in Geog and Economics at AS, he left the sixth form and went to the local Tech to do a computing BTEC, class of eight. He was pretty much on board with the change as he, too, had begun to observe how the rah-rah squad of parents and teachers fell away as the years passed, and that he was, effectively 'on his own'. He also had to swallow his innate snobbery, leaving a well-regarded sixth form to go to Tech- but there he discovered motivated, committed young people, many in hard hats and steel toed boots who were getting stuck into the workforce, and earning, with far more potential that he had had, failing at sixth form!
Thus he came out with Distinction star x 2 and a Distinction!
He's now just finished Y1 of a Softwear Engineering degree, on track for a First.
The biggie was immaturity. I knew at 5 that DS1 should have been allowed to repeat YR, him and half the boys in the class. Instead, he 'repeated' Y12.
DS2, completely different kettle of fish, he knows he's too young for uni this Sept so he's doing an Art Foundation Year prior to uni. Another boy who should have repeated YR.
Good luck, hope there's some hope in here for you!