He's quite a perfectionist and put himself under a lot of pressure. He's suffered from an ED in the past and he lost a lot of weight throughout the series, visibly so. He looked quite drawn and haggard
He was unfavourably compared to Harry Judd by viewers (Harry won his series and was more naturally talented at dancing*, had better routines) and the judges were pretty harsh compared to others who were worse.
I don't really rate Amy's choreography and she's a bit insipid. Tom tried really hard and they were slightly above average (his ballroom was generally better) but his obsession with doing well/not failing was off-putting and you could tell he wasn't handling failure well. In the sense he was somewhat stroppy about it which doesn't go over well on tv and made him look quite egotistical (Fletcher trait!). It's surprising that both Fletchers, who've been in the spotlight most of their lives lack healthy ways of dealing with personal and professional adversity. You'd think they'd be able to afford some therapy by now! Tom was clearly out of his comfort zone but he's a performer by trade, he ought to have some coping skills. Some of the McFly releases haven't done all that well, it's not been success all the way. The projects he pivoted to have all been extremely successful so maybe he assumed Strictly would be the same. I've heard he has quite a difficult/stressy personality in McFly so maybe he couldn't hide it so well on tv.
*Harry allegedly took dance lessons well before Strictly (cheating haha), sought advice from his friend Kara Tointon, the previous year's winner and trained hard in the gym beforehand in terms of stamina and endurance. He was in it to win it and was paired with Aliona, one of the best pros imo. I don't know if Tom did any of these things - not to the extent Harry did apparently.