His biggest regret to date should be realising his reaction to that Burnley question post Bournemouth. He showed he thought he was bigger than Boro and therefore lost trust, that resonates across football. However, as implied, i think others also saw that lost trust and only the duplicitous Watford offered him a lifeline, to which the squad subsequently gave their verdict on.
I misread him, his abili-teh and he is the architect of his own downfall.