Maguire picked up a calf injury in training at the beginning of May and could not recover in time for the FA Cup final win over Manchester City at Wembley four weeks later.
Another setback on England duty led to Southgate cutting him from the final 26-man Euro 2024 squad.
“To miss out was tough,” added Maguire.
“It has been really frustrating, working all season to get myself mentally and physically at the top of my game and be in a position to feel I could really make a difference for Manchester United and my country.”
The 31-year-old earned plenty of praise last season for refusing to quit United when he was stripped of the captaincy by manager Erik ten Hag and at times dropped down to fifth choice central defender, behind Lisandro Martinez, Raphael Varane, Victor Lindelof and Jonny Evans.
Maguire made 31 appearances in all competitions, despite not starting a Premier League game until October.
United have spent £52m on 18-year-old French defender Leny Yoro, but Maguire says he’s ready for a renewed challenge for his first-team position.
“There was a lot of uncertainty over my future last year but I sat down with the manager, he told me where I was at, said he believed in me and I would get chances,” said Maguire.
“I proved last season when I got the chances I took them really well.”
United’s FA Cup final victory changed perceptions around Ten Hag and probably saved his job.
But Maguire knows the eighth placed Premier League finish was not good enough.
With Dan Ashworth now in place as sporting director and Omar Berrada the new chief executive, Maguire expects improvement but is realistic about how far United might go in the months that lie ahead.
“I don’t think we will go from finishing eighth in the league to winning all the trophies overnight,” he said.
“But I do think we are moving in the right direction now and the decisions that have been made at the top will help this club get back to where it should be.”